If a 51% attacker makes an invalid transaction, you actually don't want to accept it just because they have more hashpower; if you did then 51% attackers could arbitrarily steal funds. If you don't accept invalid transactions, then you're using external knowledge of current validation rules.

Also, if you really assume that only the chain with the most hashpower is the valid one, then you have to ignore all other blockchains. And even then, you're assuming you know the correct hash function; given ASICs it's not practical to know what hash function requires the most economic effort just by looking at it.

Knowledge of consensus rules is the knowledge you’re building consensus upon. It’s a given.

But you have to get that knowledge, and the specific software to implement it, from some source other than the blockchain. In practical terms, it barely changes the security model if the download for new users includes a known blockhash from several months back, instead of only the hash of block zero.

Barely but importantly. With bitcoin for every user it is still feasible even on raspberry pi to download and verify the whole chain for themselves. Not so for ethereum even with high end computers. I guess that is their way to phase in PoS scam, but it has nothing to do with “preserving the security model”.

Utxo commitment is definitely a shortcut that bitcoin will eventually implement, but it can only work reliably for long timeframes especially when you have “disturbances in the force” like what happened with BCH last year.

“Known” blockhash from several months back is something that can easily be faked unless you actually fetch all the history and verify.

Ouroboros Genesis solves this problem.

nope, it doesn’t.