A similarly unanswerable, and interesting question, yet one you never see asked is: if Fischer, Kasparov, Carlsen, etc were born in the year 1837, and discover chess as children with no coaching or anything, where would they have ended up on the 19th century chess food chain?
I like the intent of this question. However the correct answer lies in another question.
If Jordan, Lebron, kobe, etc were at the ymca when basketball was invented where would they end up on the food chain?
The answer to both questions realistically turns out to be nowhere becasue the first people to play a sport or game are not remembered in the least. Only once something acheives significant popularity do figureheads arise that become bigger than the game itself.
The fact is that there are more people today then yesteryear and thus greater specialization and greater natural apptitude is required to be truly great at something. People do apear to overestimate how much more the degree of this is. So you get people saying crazy things like they could beat an old master at chess, or they could have played in the NFL in the 80's or whatever delusion that suits there fancy.
very hard to accurately pinpoint a rating for chess players in the 1800's.
I will say that I personally believe that a capablanca or a fischer would be very near the very top even today.
If the question is how well the players of the 19th century played compared to those today, I don't think one can play around with hypothetical discussions about them being given the same upbringing as today or access to theory and engines in the same way as everyone today etc.