I would say that making Epic or Marathon any longer than they already are would run into unreasonableness, especially Marathon. There's just an obscene amount of turns even currently, and a lot of the time turns feel empty while waiting for Science/Production. Further, while the game isn't balanced around these settings, the difficulty already collapses with either one; adding more turns would only compound that problem for the longer difficulties. More doesn't always equal better, with turns in particular being a place where more can create a lot of problems.
Even with the new map, increasing the number of turns by 50% is a dramatic rebalancing; just compare what you can do right now in Epic versus what you can do in Normal (Epic has 50% more turns) in the same year-by-year timeframe (even with Epic's reduced Science/Production speeds) to get an idea of how much changing turn count can break the game.
This isn't even touching on the performance problems that emerge as more turns elapse, which is very noticeable once you get into the 1900's (earlier with Epic/Marathon). Why bother adding more turns when the game takes ages to load?