And they could also install a hardware keyboard. Neither of which is going to happen. Just because you can do something doesn't mean you should.
Fingerprint reader on the back is unobtrusive and quite an intuitive way to unlock a phone. I can hardly find any issue or "major compromise" with it.
A physical keyboard on the other hand makes a phone at least twice as thick, twice as heavy, and twice as ugly (although the last one is more subjective).
If I had a dollar for every time I watched someone turn their Android phone around to look for the fingerprint sensor on the back, I would have a lot more free time to post on HN.