Thanks @stwenhao a lot for your explanation! Indeed, SPV seems to be a very good model to compare. Now to the last part there's still one thing I don't understand: But once these proofs become mandatory, when nobody is anymore storing the "classic" UTXO set and Utreexo nodes are considered "full nodes", then I guess some consensus change would become necessary, right? Let's speculate the whole OP_RETURN drama leads to a "victory" of Knots, and all Xsmt Thi 2 protocols switch to fake public keys (Xsmt Thi 2 Stamps ...), causing the UTXO set to explode. That makes node owners aware of Utreexo and they switch massively to some utreexo-supporting client. But if the UTXO proof for transactions was still not mandatory, I guess there would be a conflict between transaction senders who omit the proof (and maybe don't know how to obtain it) and nodes not providing the classic UTXO set anymore. Of course the wallet client maintainers would have eventually to implement that, but wouldn't it cause disruptions if major wallets like Electrum lag with that implementation and many people would have difficulties to send their transactions. I imagine a new transaction version with the UTXO proof integrated, implemented via softfork, would be the way to go? BTW: is there currently an Utreexo implementation really ready for productive use? There seem to be indeed projects like utreexod and Floresta (Electrum server). But these seem to be small hobbyist projects and for Core (and other major clients like Knots) I haven't seen any announcement yet, or am I wrong? That whole bunch of questions was what I meant when I asked if Utreexo is already "ready" or "something for the 2030s". Because a Knots "victory" in the OP_RETURN drama is definitely not impossible, but the consequences could be devastating. So it's not a bad idea to have a plan ready to upgrade the network to it ...
Thanks @stwenhao a lot for your explanation! Indeed, SPV seems to be a very good model to compare. Now to the last part there's still one thing I don't understand: But once these proofs become mandatory, when nobody is anymore storing the "classic" UTXO set and Utreexo nodes are considered "full nodes", then I guess some consensus change would become necessary, right? Let's speculate the whole OP_RETURN drama leads to a "victory" of Knots, and all Xsmt Thi 2 protocols switch to fake public keys (Xsmt Thi 2 Stamps ...), causing the UTXO set to explode. That makes node owners aware of Utreexo and they switch massively to some utreexo-supporting client. But if the UTXO proof for transactions was still not mandatory, I guess there would be a conflict between transaction senders who omit the proof (and maybe don't know how to obtain it) and nodes not providing the classic UTXO set anymore. Of course the wallet client maintainers would have eventually to implement that, but wouldn't it cause disruptions if major wallets like Electrum lag with that implementation and many people would have difficulties to send their transactions. I imagine a new transaction version with the UTXO proof integrated, implemented via softfork, would be the way to go? BTW: is there currently an Utreexo implementation really ready for productive use? There seem to be indeed projects like utreexod and Floresta (Electrum server). But these seem to be small hobbyist projects and for Core (and other major clients like Knots) I haven't seen any announcement yet, or am I wrong? That whole bunch of questions was what I meant when I asked if Utreexo is already "ready" or "something for the 2030s". Because a Knots "victory" in the OP_RETURN drama is definitely not impossible, but the consequences could be devastating. So it's not a bad idea to have a plan ready to upgrade the network to it ...

Thanks @stwenhao a lot for your explanation! Indeed, SPV seems to be a very good model to compare. Now to the last part there's still one thing I don't understand: But once these proofs become mandatory, when nobody is anymore storing the "classic" UTXO set and Utreexo nodes are considered "full nodes", then I guess some consensus change would become necessary, right? Let's speculate the whole OP_RETURN drama leads to a "victory" of Knots, and all Xsmt Thi 2 protocols switch to fake public keys (Xsmt Thi 2 Stamps ...), causing the UTXO set to explode. That makes node owners aware of Utreexo and they switch massively to some utreexo-supporting client. But if the UTXO proof for transactions was still not mandatory, I guess there would be a conflict between transaction senders who omit the proof (and maybe don't know how to obtain it) and nodes not providing the classic UTXO set anymore. Of course the wallet client maintainers would have eventually to implement that, but wouldn't it cause disruptions if major wallets like Electrum lag with that implementation and many people would have difficulties to send their transactions. I imagine a new transaction version with the UTXO proof integrated, implemented via softfork, would be the way to go? BTW: is there currently an Utreexo implementation really ready for productive use? There seem to be indeed projects like utreexod and Floresta (Electrum server). But these seem to be small hobbyist projects and for Core (and other major clients like Knots) I haven't seen any announcement yet, or am I wrong? That whole bunch of questions was what I meant when I asked if Utreexo is already "ready" or "something for the 2030s". Because a Knots "victory" in the OP_RETURN drama is definitely not impossible, but the consequences could be devastating. So it's not a bad idea to have a plan ready to upgrade the network to it ...

Thanks @stwenhao a lot for your explanation! Indeed, SPV seems to be a very good model to compare. Now to the last part there's still one thing I don't understand: But once these proofs become mandatory, when nobody is anymore storing the "classic" UTXO set and Utreexo nodes are considered "full nodes", then I guess some consensus change would become necessary, right? Let's speculate the whole OP_RETURN drama leads to a "victory" of Knots, and all Xsmt Thi 2 protocols switch to fake public keys (Xsmt Thi 2 Stamps ...), causing the UTXO set to explode. That makes node owners aware of Utreexo and they switch massively to some utreexo-supporting client. But if the UTXO proof for transactions was still not mandatory, I guess there would be a conflict between transaction senders who omit the proof (and maybe don't know how to obtain it) and nodes not providing the classic UTXO set anymore. Of course the wallet client maintainers would have eventually to implement that, but wouldn't it cause disruptions if major wallets like Electrum lag with that implementation and many people would have difficulties to send their transactions. I imagine a new transaction version with the UTXO proof integrated, implemented via softfork, would be the way to go? BTW: is there currently an Utreexo implementation really ready for productive use? There seem to be indeed projects like utreexod and Floresta (Electrum server). But these seem to be small hobbyist projects and for Core (and other major clients like Knots) I haven't seen any announcement yet, or am I wrong? That whole bunch of questions was what I meant when I asked if Utreexo is already "ready" or "something for the 2030s". Because a Knots "victory" in the OP_RETURN drama is definitely not impossible, but the consequences could be devastating. So it's not a bad idea to have a plan ready to upgrade the network to it ...