Thunderbird isn't acquiring K-9 to kill it or merge with an additional product, they're buying it so they can increase its development speed and then put their name on it. That seems to me like about the best acquisition goal that they could have -- basically saying, "what exists is good, instead of building our own thing let's just pour resources into what exists."
It doesn't seem that conceptually different from what companies do with products like Blender, where sponsoring more development on an existing product is better than rolling an in-house version. The main difference being that K-9 is not independent now, the companies that do this with Blender don't end up owning Blender.
So with any acquisition there's some cause for caution, but overall I am inclined to be pretty optimistic about this. I use K-9 today, and I would like to see K-9 get more resources and to get more parity with other clients. I'm a little bit hesitant about my primary email client on my phone entering an experimental phase where its interface is going to change a lot. Again, concerns, but overall this seems like actually pretty good news.