The bigger the max block size is, the more spam they could put in a single block. And that "heavily invested in" part may apply to Bitcoin, but definitely not to some altcoins, that are mined, and then exchanged into Bitcoin, just because mining altcoins and selling them is often more profitable (as long as mining is not fully decentralized, and as long as miners rely on centralized pools). Another thing is that if someone doesn't like some altcoin (like BSV, because of Craig), then that kind of spam can be always executed to make it even more centralized, and put even more weight on a few nodes that keeps the whole thing alive. Also, when the block size is not limited, then miners could generate that spam in a deterministic way, then the cost of storage is quite low for the attacker, but quite high for all other nodes, because they cannot compress data, without knowing the seed that was used to generate spam.