I agree that the drift in chess Elo ratings is slow enough that we can say Bobby Fischer at his peak would still be a strong grandmaster today (if he were given some time to study developments in opening theory). But I don't think we can say whether he would land at 2700 or 2800 in today's Elo scale.
This is within the realm of reasonable opinion, but I'd tentatively say it's an uncommon one--I thought it's generally agreed that the very best players of that era didn't lag contemporary players in terms of skill, only theory. I may be wrong, though.
Chess has developed significantly over the last 50 years. The best players now can be reasonably expected to be much more skilled than the best players of 50 years ago, even if you discount opening theory knowledge.
