By trying to appeal to this "mythical" broader audience, in doing so they spurn or otherwise upset and marginalize the worth of those that got them to where they are now.
The developers need to decide what they want their games to be and apply to them that focused vision, instead of constantly trying to shift it into something it is not, in attempting to snag an audience that never cared for their games in the first place. Skyrim should have been an RPG, what it is now however is essentially a mediocre action game with poor RPG elements.
There is a serious problem in my opinion, if your long time fans can take a step back, and look at the newest game you made, and begin to think for even one second, that the type of game that is being made, no longer resembles what initially interested and drew them to the company in the first place.
Honestly, why should they care about their old fans? For every one they lose, they can gain 5 more by changing up their m.o. That makes sense from any perspective. I'm looking for logic in your post, but all I see is heartfelt loyalty issues. There's a reason loyalty and honor-based feudal systems collapsed all over the world. They don't grow.
The broader audience isn't mythical. It's there. Look at the wii. Not a single hard core gamer bought one, but it's one of the best selling consoles of all time. Why? Casual gamers, family types, children. That broad audience is not a myth.
There's no point in pining over what Skyrim
should be. It should be whatever Bethesda thinks will sell. If you don't think it's what you want, then don't buy it, regardless of how much you liked the former games. The problem lies in your expectations, not in those of the developers. If there's a hole in the market for a certain type of RPG and Bethesda doesn't fill it, someone else will. If they don't, then it's not big enough for anyone to care about.
At that point, sadly, you belong to nobody's target audience. You can either change your expectations and join an audience that is targeted, or you can simply long for something that won't come.
Of course, some concessions need to be made, but a developer can't simply go forward and automatically assume their already loyal base will eat it all up regardless. And there is also the fact, that even should they bend over backwards to accommodate new blood, there is no guarantee these people will buy it anyways, because they had no interest in the first place even after they were catered to.
There is no reasonable expectation that the entirety of their loyal fanbase will eat up any new game that they release. In fact, most developers expect to lose a certain percentage of their former fans each generation, simply because the new game is not a carbon clones of the old game with updated graphics. But at the same time, they don't care. Their effort is not to drive away loyal fans, it's to attract new ones. So long as that works, then developers will continue to do it, "loyal base" be damned. If anything, this "loyal base" is more mythical than the broader audience they would like to appeal to every generation.
Skyrim is the most sucessful TES game to date. If it had been Morrowind 2.0 with new graphics, it probably would have only been a bit more successful than Morrowind was. Why would a game company want that? That doesn't make sense from any perspective.