This is a bit of a tangent but you mention that Bitcoin's price would be unaffected. Personally I don't foresee that. It seems to me that when knowing that an attack is underway, some Bitcoin investors would already start to get uneasy, and some would sell their bitcoin. And since this will cause the price of Bitcoin to drop, the 51% attack will then only be easier to afford in theory, as the mining rewards would be decreased and some of the honest miners therefore ought to fall off. This could create a feedback loop where, once the attack is looming in the near future, more and more Bitcoin investors will migrate elsewhere, potentially to Ethereum.
if the network knew the hash power was to get 2x competition.. meaning older miners get half as much sats... its the same as a halvening event in regards to miners prospects.. we already seen miners re-evaluate global bottom value change from $25k per btc to now being ~$50k bottom due to reward halving
as we have already discussed the implications of a multi-month ramp of of batches to get a colluding group to 2x the hashrate to get 51% of the network.. to match/beat the honest half of the network.. those honest miners would take that opportunity to buy more coin cheap at the $50k-$70k(current) knowing that the impending hashrate /difficulty growth and reduction in sat rewards would cause the market to INCREASE
in that same time exchanges would look into solidifying their deposit algo's to not allow new deposits to trade unless it meets X confirms. and then even when trading on exchange mysql databse balance. not allow withdrawals for X time to ensure that any nafarious pool trying to re-org the blockchain would need to re-org many many many blocks back.. meaning (as mentioned before) the colluding group would need far more then 51% of the network to be able to re-write blocks very far back
if you run the math of average block speed and how far back a 1% speed benefit means in reality to going backwards and then moving forwards to catch up and over take.. 1% benefit over honest network wont get the colluding group to go that far back. so services can just enforce the "6confirm rule" or a 72 hour wire transfer delay to evade colluding pools from trying to double spend