Would the versions with replacable backcovers improve repairability?

I would say yes. Especially with the m7. In my opinion the hardest part of a repair is getting the back case off with out doing any sort of damage. It would definitely improve its repairability. The next thing that makes the HTC ones hard to work on is the amount of tape they use and how many flex cables there are. The tape and cables are a mild inconvenience compared to removing the back case in my opinion. I enjoy working on the HTC ones mainly because they can be a challenge, but the case on the international m7 is a pain, so I'd rather work on the m8 and m9. They are improving in time, thankfully, when it comes to how easily the back case is to remove.