Hm, these markets in Shenzhen seem to offer different stories. Youtube is full of those, you can replace/upgrade displays, memory, storage of your phone or even assemble your own.

And in my country you have these little shops as well, where you pay a premium, but you dont have to buy a new phone.

So, unless you really want to fix something very integrated, i am rather positive about the situation.

And from my own history, mainboard was never really a problem, it was always something attached or plugged or solded onto it.