When I can order a RK3399 on aliexpress and get a PCB fabbed in china for a few dollars, what’s stopping me?

Just because you don’t know how doesn’t mean other people shouldn’t.

Yes. Plenty of us already doing this and things like it.

The closest i can think of to that is buying a bunch of broken shiftphone/fairphone and making a new phone with the working parts.

But its not really choosing the design but instead a type of repair with spare parts.

in that sense other phones can also be repaired this way but they need a lot more skill like ungluing without breaking.