So you picked a feat you didn't like? Learned a spell you aren't a fan of? That's okay, you can go to town and pay someone to retrain you in a new feat or spell. Hey great. Now you just have to forget the old one. What? Why does that work? I paid this guy to teach me, not stick a hose in my ear and siphon out my old skill.
At first thinking about this, I was approaching it from an entirely "real world thinking" approach. Like yeah but that's not how getting taught things works. I learn to play the piano I don't forget how to do origami.
But then as I thought about it, I saw it from a better perspective. Normally you buy one of a set of feats and you can't ever get the others, but what if it was the same thing as getting a magic item. You pay an NPC some gold, you get an interesting mechanical boost your character gets to use. How is that different from buying a magic amulet? The game is already full of this kind of stuff to spend your treasure on. This would be just one more thing you could invest gold in to make your character better. Especially now that magic items are much less of a factor. And that way you don't have to learn one thing at the expense of another. You get to keep both.
You can already hire people in town to teach you formulas or wizard spells. So instead of 'retraining' you're just doing this. You're finding another elf who has keen hearing and training blindfolded with him until you've honed your own inner gift. Great. You paid for it. Enjoy it. No need to 'pay' for it again by mysteriously forgetting the innate cantrip you could always cast.
I don't think we need retraining at all the more I think about it. Am I overlooking something? Is there some downside to all this I'm neglecting? It just seems like a better way to handle it.