Once we attach a single peak rating to every player, it’s only natural to start comparing across eras. While it’s always fun to do so, I’m not sure
any rating system allows for useful cross-era comparisons in tennis. Elo doesn’t, either.
What you
can do with Elo is compare how each player fared against her competition. In 1990,
Helena Sukova achieved a rating of 2123–exactly the same as Barty’s today. That doesn’t mean that Sukova then was as good as Barty is now. But it does mean that their performance relative to their peers was similar. The second tier of players was considerably weaker thirty years ago, so in a sense it was easier to achieve such a rating. At the time, Sukova’s rating was only good for 11th place, far behind
Steffi Graf’s 2600.
Thus, Elo doesn’t allow you rank players across eras unless you are confident that the level of competition was similar–or unless you have some other way of dealing with that issue, a minefield that many researchers have tried to cross, with little success.