I don't see why Bethesda can't capitalize on both fan-bases. Give Obsidian a budget necessary for a turn-based isometric cRPG and reap the profits.
It's not about the highest amount of money they can earn, but rather how much budget they can put out and how much of a profit they will gain from it.
Bethesda knows Obsidian or InXile could do this.
Bethesda has the money to fund a smaller budgeted game and not worry if it is a flop (since it is a far lower budget than a triple-a title would be).
Bethesda knows that there is a split in the fan-base.
To me, it'd be logical to at the very least 'try once' and see how it turns out.
So yeah, if Obsidian is allowed to create a Fallout game I would hope that they'd (if given free reign over the development) return to isometric turn-based combat with a cRPG design. But I wouldn't want to force Obsidian to do anything. I'd rather they be allowed to do what they want to do as that'd probably give us the best product they have in mind.
Again, it is about the budget and the profit margain. A triple-A title may earn a higher number but in % how much did it profit?
And at the end of the day, if a cRPG by Obsidian would be profitable for its budget, what'd be the harm in allowing them or other studios that knows how to develop cRPG's to develop more of them?
For me, the changes Bethesda made for Fallout 3 were detrimental to Fallout's future. Even FNV which takes a few steps back into the right direction and even adds some things the earlier games didn't have which I really like it did not feel like a true Fallout game. It'd be absurd of me to demand or even 'ask' that Bethesda themselves return Fallout to what it once was. That ain't gonna happen. They're gonna keep making their The Elder Scrolls games with a Fallout paintjob. But it is not unfeasible to ask that they cater to fans like me as well as the new ones, and it can be done.
You who like Bethesda's Fallout will still get your Bethesda Fallout games. I don't see what harm there'd be by giving the cRPG Fallout fans a game.