"Private Mempool" is such a nice euphemism for "closed source submission service". They're sort of BS even though they're completely within the rules. Trying to find the positives, I feel it highlights the freedom, flexibility to innovate, and ability to contract at will when you run the miner and the pool, and a bunch of miners are watching them for signal. And watching El Salvador's Lava Pool too, the first PPP (public private partnership) miner-pool, to see how successful they are. Maybe it breaks the now dominant pools down into more strong independent miner run pools instead of just a miner directing hash to a 3rd-party hashpower blob, and the remaining 3rd-party pools are more plebeian pools, not squeezed so tight on a pie-chart that they're invisible, and they're adding txs from the network like always. Marathon Pool running Slipstream is small (2.5% of hashpower) but they fired a shot here and their public competitors look boring to investors, even if the product Slipstream doesn't add up to anything.

I don't think it's a threat to mempool propagation or small miners with good energy or challenges node checkpointing, because I think it's going to be a thing for very large transactions with many outputs and complexity. Or one-off stuff that needs a confirmation window. Or future dated stuff that wants to lock-in an advertised rate today. I'm hoping this all acts as tx hand holding that helps normalize/incentivize much larger transactions (not Taproot Wizards shitcoinery) and thus helps scaling incrementally.

I agree strongly with your last part. And there always needs to be a reality where there's enough miners that a tx (with a suitable fee) will eventually be confirmed regardless of size or locale.