aenemic said:
take a much bigger franchise for example... how about Marvel comics? all their different superheroes have appeared in countless different forms, but all of it still belongs to the one and same franchise.
That's not even remotely the same type of "franchise".
Game franchises that changed and thrived/were well received initially are the minority. The ones that succeed are usually those that make changes to mechanics in order to better support the core gameplay concept (GTA) or those where the franchise staple is reinvention (Final Fantasy), but generally if you actually abandon the core concept than normally the perception is that you messed up, and the game often fails (X-Com).
Bethesda got away with it because of loads of money and because most people understand that "RPG" is an umbrella term genre and TES and Fallout are wildly different types of RPGs.