^Reading your post and thinking about it for a bit, it is a miracle the game was made in the first place.
I both agree and disagree. I agree, in that in an absolute sense, it would have seemed unlikely that a TW game covering that time period would ever be made.
On the other hand... Once CA determined they were going to make a "Saga" title (their first) -- and one in which they were planning to experiment with different gameplay features, mechanics, and/or concepts -- then the British Isles during the time of Alfred the Great was probably a better choice than most.
The time period enjoyed a certain level of recognition (thanks at least in part to Bernard Cornwell's
Saxon Tales novels & TV series), so it's not as if the choice was completely out of left field. At the same time, however, it was still obscure enough that should the game turn out to be a failure, it was likely that that the fanbase wouldn't be as disappointed or upset -- or at least, that a smaller percentage of the fanbase would feel less put out by an underwhelming Total War title. (Contrast Troy and the Trojan War, which is far better known.)
And in the meantime, CA gets to find out which changes worked, which ones didn't, and which changes *could* work, but need to be tweaked/adjusted before being added to future games. If ToB turns out to be a big hit as well, so much the better; but I honestly doubt they were ever expecting that.
Ultimately, I suspect
Thrones of Britannia was the result of a bit of serendipity: CA wanted to test some new concepts & mechanics, and recognized that they realistically couldn't do so without actually putting out a new game. That being the case, why not indulge their inner history nerds (and possibly a bit of national pride) by making a "smaller" (relatively speaking) TW game centered around a lesser-known period, yet still familiar to (and popular with) a certain percentage of their fanbase?
I'm sure I've got a lot (and maybe even most) of the details wrong in my speculation here, but I'm willing to bet I'm at least in the ballpark.