Nexus devices are the closest you'll get - can be unlocked by a vendor supported, fully reversible ADB command.
After looking for a while into various phones, I ended up getting a Sony Z3+ -- as it combines many things I want (water proof, memory card slot) with a pretty strong commitment from Sony:
http://developer.sonymobile.com/open-devices/
Ironically, I've yet to actually experiment with different roms and other software on my Sony phone, but there it is.
The code "protecting" the bootloader is good enough that it could be very, very difficult to unlock your bootloader without official support.
I feel this is a little bit different from the "zune drm hole" where you "buy" (rent) content, but might loose all access once the provider goes under/closes shop. (See also: the case of Amazon recalling copies of "1984" it had sold as an e-book without a proper license).
What if I buy it from eBay 5 years later?
I can reversibly root it?!