Does Utreexo have this issue? If done as a soft fork, it reduces the UTXO set storage requirement to about 1kb. It would make UTXO “spam” completely irrelevant. It has other benefits too, e.g. being able to validate the blockchain quickly with very little RAM.
This comes at the expense of making the input (witness?) bigger since the spender needs to prove that a coin exists.
In the very long run, e.g. 1000+ years, having every node store an ever growing unprunable UTXO set doesn’t seem ideal, with or without spam / dust.