If you need non pruned node, you will store spam regardless of how it's encoded. Your node would be slower though, using a larger data structure is more computationally complex.
Many people can run a pruned node.
Op_return spam vs utxo set spam - clear winner for op_return in both cases.
Mining pools that get more btc from fees will gain hashrate, not lose it. From the off band tx they will include only those that pay more than those in the mempool. And since you can mine each block anonymously, it doesn't matter for them.. If I have a miner, I want most sats per hashrate.