I don't see where's that even relevant? You just see the transaction, until they mesh it. You don't know who and there is no accountability if there's abuse.

It's the preferred method of payment for scammers, so visibility means absolutely nothing.

Reminder that 51% were even possible in the past. See https://en.wikipedia.org/wiki/GHash.io

Seeing the transactions weren't a solution then and aren't a solution now.

Assuming there is a detected 51% attack, what would be done against this? Spinning up more nodes to counter attack is not an incentive for other "good" (whatever this means) miners because keeping unused mining capacity at hand is equal to losing money.

Detection is worthless without applying counter measures.