The problem isn't that much with wallets which are synced regularly, where normally the UTXOs are saved in the wallet database too, even if it's a SPV wallet, so the information needed for the UTXO proof indeed should be always available. Instead it could appear if we have an old HODLer, who has not touched his coins nor synced any wallet in years, and then wants to transfer them. If he isn't able to connect to an archival node who has stored the whole blockchain, or alternatively the UTXO set, he wouldn't be able to move the coins.

Of course this is a step further away than the "Knots wins and triggers Utreexo adoption" scenario, it's a scenario where even true full nodes are scarce and most nodes are pruned. This could however happen after some massive injection of illegal material in the blockchain via the "fake public keys" model. It's true that it is unlikely that the number of full nodes able to provide the UTXO is so low that there are really problems to connect to one. However, if it comes close to that scenario, it could be used for FUD and maybe some economical attacks.

I agree with your conclusion that users should be made aware to save UTXOs when they receive a transaction. I hope however that once Utreexo adoption begins to increase, Core implements it "officially", preferrably with a transaction format update.

BTW there are three draft BIPs which came out relatively recently and have just 2 weeks ago been asigned the BIP numbers 181 to 183 (but are still not official):

- Utreexo accumulator specification (BIP 181)
- Transaction validation (BIP 182)
- P2P networking changes (BIP 183)

The second and third BIP are just what I was referring to - the additional information which has to circulate about UTXOs.