Apparently only infants needing blood or tissue that require an oops you are Not the daddy have non compatible parts...but once the magic of puberty hits almost everyone has interchangeable parts
You know, if they did that for us viewers who want Stefano dead, I'd love it. Reveal that Rolf is (bonus/extra/especially) crazy, thinks he IS Stefano, and HE was the one on that Bench in Europe...a little Easter egg for the fans.
The consensus (here and elsewhere) seems to be that the decision was made to kill/erase Stefan BEFORE anyone knew how fans would react to the recast/and Gabi pairing. (Given the show's insane tape delay, that's not a stretch.) These scenes would have been written 10-11 months ago and the negotiations with the actor...at least a month or so before that, I would guess?
So now you have a reborn character in a popular pairing, and you've tossed him in the dustbin, because you had to make a decision about what who'd be on contract in 10/2019 back in late summer 2018 based on how much we hated Stefan then. Not sustainable!