Yes, fake pubkeys increase the utxo set, which means that other than taking space in the blockchain, they also increase the space needed in the utxo DB. Not in RAM, as they will be pretty soon placed in disk cache never to be fetched again, since they cannot be spent, but still.
Storing the same amount of data in fake pubkeys costs (depending on feerate) up to 30 times (if you consider 1sat/vB as feerate floor, even more for lower feerates) the cost of OP_RETURN, and way more than that compared to witness if you consider the 75% discount.
So yes, fake pubkeys increase the space requirement for full nodes, but are much more expensive than that for spammers.
Game theory says that if you do not give the possibility to store garbage in huge multiple OP_RETURNs, and maybe patch the possibility to do that in witness, spammers will have a much harder time spamming.
