- Block propagation. Slower if the nodes have many different policy settings (as far as I understand it).
I think this is the only noticeable effect, and yes, your understanding is 100% correct. Different policy settings cause an overall public network performance degradation. If you don't store X data in your mempool, then when you receive a block that includes X, you'll still need to verify it -- meaning you’ll have to download it anyway, but only on demand. That’s very different from having it already downloaded and propagated a dozen times. It’s like a torrent client forcing you to download and seed the entire network, including files you may never need.
Mind you, this propagation delay won’t affect most miners, since they will certainly relax their settings. And more importantly, mining pools don't rely on the regular P2P network (the one you and I use) to relay blocks between them. They mostly use protocols like FIBRE, which are obviously much faster than using the public network.
bribe [2] miners directly, increasing their cost. The crucial question here would be, how high is the average cost of this effect?
- Centralization risk due to direct miner bribing: Big miners and pools can increase their income due to direct bribing. This can increase the difficulty and price smaller miners/pools out of the mining game (may be a small effect, but it probably exists).
More transactions in blocks = more profit for every miner, no exceptions. If a bribed pool drops a standard transaction to make room for an OP_RETURN transaction, that standard transaction doesn’t just vanish -- another miner will pick it up. So the overall mining revenue increases with more transactions, regardless of type.
Now, since spammers may be forced to reach out directly to mining pools, those pools will likely charge them more. And honestly, isn’t that a good thing? Don’t we want spammers to be paying higher fees so they eventually get tired and give up?
And let's not forget: miners are also part of the community. Individual miners can pressure pools to reflect their preferences -- like saying "don’t include those trash transactions or I’m leaving your pool." That’s already happening with Ordinals.
You would think a pool like Luke’s Ocean, which bans all sorts of transactions (and as a result earns less, ignoring variance and fees), would be irrelevant by now. But nope -- they've managed to gather nearly 6 EH of hashrate and are still growing!. That tells you there are plenty of miners who don’t want their hashpower used to mine what they consider spam.