Looking at evidence of the other games in the elder scrolls series it seems ok to suggest that the TES series is becoming more and more simplistic (mainstream) for example in daggerfall we had dynamics such as boats and a banking system and alot of factions.
Daggerfall didn't really have boats. You could buy a ship... which was basically just a house shaped like a big boat. That's about it. The banking system was nice but really added nothing to the game (money's as trivial in Daggerfall as it is in all the rest, so being able to take loans and deposit cash really doesn't matter). And as for having a lot of factions... no, not really. Not at all. Daggerfall has a pretty small handful of factions that were copy-pasted with very slightly different names several times.
Then in morrowind we lost the banking system and some factions as well plus a smaller map. I have to confirm it but i think we also lost the ability to buy horses in morrowind as well i dont remember.
Already covered the banks and factions. The map was smaller, but on Daggerfall's map very, very little was unique or distinguishable (we're talking maybe a few landmarks in a few of the largest cities and pretty much nothing else). Morrowind's map being smaller isn't necessarily a bad thing given that it's not just a bunch of square buildings and some bumpy terrain with absolutely no roads or other details (aside from trees and a puddle or two) whatsoever.
Then came oblivion, i great game in my opinion but even more main stream. we lost all factions except for four, creativness in cities, various spells were gone such as slow fall and levitation and other aspects.
Once again, you need to consider what came along with those changes. Oblivion has fewer factions, but they're all far,
far more interesting and diverse than they were in Morrowind as far as the quests that they offer, and quests in Oblivion in general tend to be considerably deeper than Morrowind's "go here, kill this, get this, go here". Not that Oblivion doesn't have those quests, but it'll throw twists in here and there - you'll be doing the killing with a group of men or you get to choose how to kill the target and get a bonus for killing them a certain way, the thing or person you need to find is in another dimension entirely, that sort of thing. You can criticize it as mainstream for having less, but it's pretty obvious that the drop in quantity of content was done for the sake of increasing the gameplay value of that content. Slow fall and levitate, on the other hand, were removed specifically because they wanted to actually
have cities - Oblivion's engine simply isn't very good and couldn't handle large crowded outdoor areas like that on the overall map, so they had to handle them as fully separated cells. That meant they had to remove spells that you'd be able to exploit to float over the walls of those cells, since... well, there's not really anything over those walls. And as for creativeness... well, that's not really a backstep from Morrowind, just a step back towards the norm for the series. Arena and Daggerfall were both pretty standard high fantasy for the most part, and Morrowind's a variation from the norm more than an indication of the standard.
All that said... Bethesda has pretty much always aimed for the "mainstream" with their RPGs. Arena came out in a time when fairly simplistic D&D-style dungeon crawls were very popular and when FPSes were on the rise, and it tries to provide a blend of those two. Daggerfall arrived when, for RPGs, the expectation was for something believable, and that's what they tried to do. Morrowind came along when the focus was on providing a focused experience within a relatively unique fantasy world and that's what that game gave us, and Oblivion... well, I'd go as far as to say that the classic RPG died before Oblivion, and Oblivion is a pretty clear reflection of that.
Bethesda's games have been niche, but don't fool yourself into believing that "niche" is what Bethesda's ever been aiming to be. They've aimed for what were fairly large markets with each entry in this series, and with varying degrees of success. I'd imagine they'll do the same with Skyrim, but I really don't have a problem with that.