How difficult is it to replace the keys on the Apple Wireless Keyboard?
That older model has key caps that are far easier to fix than the new low profile keyboards. 

They are still not easy and by the time the keys wear out, the plastic scissors are likely equally worn or fragile. You’ll probably want half a dozen “donor” keyboards for parts if you decide to DIY repair these era of keyboards. But with a skilled technician, what you ask is doable without special tools and large stockpile of 100% new parts.