user3423: Take the best player from the 19th century, at his prime, and imagine he started playing tournaments today. He can't learn new stuff.
What Elo would he achieve?

user291: Ken Regan's research (e.g., Understanding Distributions of Chess Performances - PDF link) suggests that the best players of the 19th century were playing at a level in the 2400-2500 range (see the table on page 9 of that paper).

3423: <p>Take the best player from the 19th century, at his prime, and imagine he started playing tournaments today. He can't learn new stuff.
What Elo would he achieve?</p>
 291: <p>Ken Regan's research (e.g., <a href="http://www.cse.buffalo.edu/~regan/papers/pdf/RMH11.pdf" rel="noreferrer">Understanding Distributions of Chess Performances</a> - PDF link) suggests that the best players of the 19th century were playing at a level in the 2400-2500 range (see the table on page 9 of that paper).</p>
