The issue with implementing the current portal rewind button is that it doesn’t erase gains. It could create quite a mess of random points and gains and break stories. (Authors frequently have to ‘reset progress’ and/or use the ‘picker’ option to test effectively.)
I imagine the code to erase what was gained in just a few lines could be pretty complicated.
…For the record, though, I would love that myself. If they could find a way to implement it well. I’ve hit my thumb down too quickly more than once.