I don't think anyone advocating for fixing filters expects to block all spam. Spam/arbitrary data will always be a part of Bitcoin and why OP_Return exists. Handling inscriptions (yet another method of storing arbitrary data in Bitcoin, certainly won't be the last) by subjecting to the same OP_Return filters is fair.

Filters are only meant to discourage spammy behavior, it doesn't stop spam. A network with participants running filters forces "spammers" to collude with miners to take on all the risk of constructing blocks at risk of being orphaned (since nodes won't validate txs which don't propagate beforehand). Nodes with filters will validate spam blocks just fine, just marginally slower than blocks w/ tx already all validated beforehand (propagated properly)

Filters don't cause forks, miners which want to take the risk to mine blocks containing txs that don't properly propagate across the network do