Obviously assessments don’t change as often as they once were, simply because the power of computation has increased. The machine simply ‘sees’ much more than a GM in the past would have. That doesn’t mean anything, as the present computers’powers of computation will be a laughing stock for future machines, and present assessments may change drastically.
‘Solving chess’ only would offer the definitive answer. Until then we are left with guesses, which despite looking stronger than those of the past, are still a far cry from the final verdict, if you look at the power of computation of present computers which, although impressive, are still able to look only at a small fraction of all the variants. And that's regarding computers! Imagine how small the humans’window of seeing the big picture is...
My point is that the beliefs (to be more specific, beliefs that moves are good) of very strong chess players are very probably mostly true, and aimed primarily at achieving that truth.
Do you disagree? If you feel inclined to do so bear in mind that every position and every move has a precise value 0, 1/2 or 1 (the value of a position is the max of values of the moves if the player has white or the min if he has black, of course), and that the empirical evidence is that most (not all, of course) positions have multiple optimal moves. I say this because the success rate of players in picking moves is helped by the fact that usually there are multiple correct answers.
Right there: ‘very probably...’—that’s a belief still. When you say ‘optimal moves’ it’s the same: ‘optimal’ for now.
No, that's not what I meant. What I meant is that it is highly probable that a large majority of moves played by very strong players are optimal in the absolute sense of a 32-piece tablebase. This is a judgement call: it is not possible to prove it: doing so would be as difficult as solving chess.
As an empirical hint of this, note that as standards of chess have risen, genuine refutations that change assessments remain the exception rather than the rule. This remains true with engines that have extremely high ratings to help with analysis. Games are won by occasional errors, often in positions where a player has made analysis relatively hard for his opponent.