Appreciate the first statement. You're right about my comment regarding recreating history, it's not exactly what happens in a TSL game.
For me, and perhaps others, the fun is a kind of what-if. For example, the Roman empire didn't fall and instead is competing with the Aztecs to colonize lands east of the Mississippi (what I'm doing right now). My next game next likely will be if the Māoris settled Australia and then moved north. Will they be able to sustain an empire against India and China?
All of that goes away with forced civ switching, for no reason that I can discern.