I don’t think we can assume that someone who has been getting 85% or 80% (or 64%) retention with SM-2 is necessarily switching to FSRS in order to maintain that level. If that’s a user who isn’t going to seek out the settings to adjust their DR themselves, I think we should put FSRS’s best foot forward and stick with the default of 90%.
I wonder where you’re thinking we would draw the line. If it’s between =0 reviews and >0 reviews, then an essentially new user who has been dabbling and experimenting for a couple days (possibly not even with real cards) before they enable FSRS gets lumped in with a user with many years’ experience. That seems to miss the mark.
A user switching over might be helped with something more along the lines of being presented with the information and offered a choice, e.g. “The default DR is 90% – your average retention for the past month is X. Do you want to stick with the default or change it?”