I think the fallacy in your argument is that you're assuming the 'mainstream' fans are the majority. What is the logic behind that conclusion?
I think I can chime in here. "Mainstream" is more or less by definition the broadest, also assumed to be the shallowest, demographic to aim at. Therefore by its very nature the mainstream market is bigger, if less inherently loyal and fanatical, than catering to a smaller, yet more loyal, fanbase. However, all mainstream inherently means is something a wide market would buy. Generally, people who see the word fill in the other, derogatory details themselves. The success of Game of Thrones and the book series it sprung from, a subcreative dark fantasy, in the mainstream market reinforces my point that what we call the mainstream is not actually as dim and narrow-minded as we often assume here.
I also wish they'd come up with a different name. Alas, it's harder than it sounds. 'Specially when you're working with an already established IP.
Name's not fantastic, but it works. Any other descriptive names would likely only make sense to longtime fans or give the wrong idea. "TES Interregnum" sounds cooler, but who outside the lore buffs would know instantly that it means second era between Akaviri and Tiber? Not to mention that doesn't immediately scream "ITSA MMO!"
What scares me most about that is you make perfect sense. If the fanbase has changed then that means that these games are no longer created for 'me'. This saddens me. I'm not angry at Bethesda, more like resigned and upset. Skyrim is still a great game. I just miss a lot of features from previous games. Edit: I should say I think that once you have a core of fans you should always cater your game for them, any additional along the way is just from spreading fame. Not to fundamentally change everything to cater for new people
Yeah agree!

, I'd hate to see people labeled as fan type II though. Labelling is just un-flexible.
Being that I write (see signature), I don't think they ever made it "for" anyone or anything but the fact that it was something they enjoyed doing that they could use to turn a profit. That may be a cold comfort, but it is a thing.
And I also agree that labels can be very inflexible, but such is the pitfall of using a syllabic language designed primarily to tell one another where the good fruits can be found and to communicate tactics when trying to outmaneuver the mammoth you hope to feed to your clan.