I think they've had troubles with every time they shook up the formula with a new major release (of the historical titles). For me it was downhill after Rome 2, which I actually greatly enjoyed even at launch, but even so I didn't think they necessarily moved too far away from the formula. Troy for sure won me back over (and is the only TW I'm playing anymore) so that's why I'm so fond of it and the upcoming spin-off, but I remain aloof towards the more mainline historical titles and have no interest in WH. I think 3K was too much of a let down and all their messaging & attitude leads me to believe they are only going to keep going in a worse direction for those games (at least for my tastes).
No doubt that's played a major role in it, but I think it's probably a lot driven by Sega's financial demands too. Not to mention just good old change over time which has toppled many great studios, see Bioware, Blizzard etc. More often than not if the studio doesn't have a singular owner that's a visionary and which can keep people on task working on that vision, then a drift happens and we see a continual decay until either the death of the studio occurs or it changes so much its unrecognizable.
I think they have moved a long way away from the original formula which is why the long-term fan base is totally disillusioned with HTW, it's because it's not HTW. The SAGA'S games are just cheap knockoffs of what the series used to be. But if you enjoy these games that's fine, there ought to be room for both but clearly CA have set fire to full-fat HTW and are now selling superhero driven mobile game TW for full price.
I think back to the classcis Rome, MTW2 and Shogun and when they were made and I remember thinking "Just imagine what these games will be like in 10-15 years time" never once did I think they'd look like this, OMG not built upon iteration after interation, but stripped back and ruined instead.
I think you're right about SEGA's financial demands, but I don't care about that I'm the consumer - I care about the quality of the product i'm paying for.