Or you could return to the original usecase and remove OP_RETURN from standardness.

I’ve never understood the bloat argument. If you have a block full of 4,000 monetary transactions, doesn’t that ‘bloat’ it just as much as 4,000 spam transactions? Is it an issue due to the amount of dust addresses spam creates? Also, uncapped OP return allows for decently big JPEGs to easily get on chain. Do other methods of embedding arbitrary data also allow for that size?

The "set" of UTXOs would be larger if packed into pubkeys (multisigs where the pubkey outputs are the actual data not "real" keys) This puts more demand on your node from a validation and storage perspective. This means it's more expensive to run a full archival node. OP_RETUN data can be pruned as it has no effect on the chainstate.