@Elroch, you are probably aware that engine ratings are not the same as FIDE ratings. If engines weren’t forced to play a variety of iffy openings against each other they would already be drawing almost every game against each other.
Here’s a thought experiment: What score would Magnus Carlsen be likely to earn against the strongest engine in a 20 game match where he was awarded $10,000 for every half point?
Theoretically, 4000 isn’t possible in chess because it is a draw.
That is not a valid inference, any more than someone watching Karpov and Kasparov play 48 games without resolution could infer than 3600 was impossible.
The highest theoretical rating is somewhere under 3600. The statistician Kenneth Regan postulated that 3571 is probably the highest an engine can achieve.*
He appears to be wrong on this one. This sort of rating is being achieved now (some estimates are that top engines on fast hardware are over 3600 on the scale extended from the human one, and ratings continue to progress by a few tens of points per year. Computational power still provides higher quality for alphazero and Stockfish (for a perfect engine, doubling processor speed would provide insignificant additional strength, of course).
But a 3571 would destroy a 2571 as easily as a 2571 would destroy a 1571.
Not sure what "easily" means but, by definition, they should get a similarly overwhelming score.
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* https://cse.buffalo.edu/~regan/papers/pdf/Reg12IPRs.pdf
I remember this paper now. With all due respect to him, Regan makes a rather absurd assumption at one point (it relates to Kramnik). Clearly he would come to different conclusions given 8 years more data.