Autocrypt is the middle ground Thunderbird should have implemented (and which Enigmail used to offer). Email is here to stay, so encryption by default won't happen as long as the PGP standard is used as designed (trust levels and all). Autocrypt improves all that horrible UX, including secure key transfer or rotation, where you can keep doing your own key management if you wish, but you have to do nothing more than enable Autocrypt if you don't. It's a mystery why Mozilla didn't push this, seems a perfect fit for them to empower regular web users.
From what I've read, PGP for Thunderbird is just a first step. My guess is that they chose the easiest/quickest path for first implementation, but I think we'll see more facilitation of encrypted email in TBird in time.
This makes sense to me. I've been trying to get friends, family and colleagues to encrypt email (hell, even signing would be a step!) for about 3 decades, now, and have basically thrown in the towel. So anything that affords a small toe-hold to begin the painful process of building the necessary network effects for the idea of encrypted email to gain traction is a good thing.
