But again, if the game is balanced around mana/stamina regens, then they shouldn't have a need for a class that is continually restoring these resources. Not only that, but we're also pretty much forced to bring certain classes along for specific actions. Without a rogue in your party, you'd be unable to pick a lock to rescue Leliana, for example, without a warrior, you can't bash down a gate to capture a fort, and without a mage, you can't energize a bridge to cross over it to seal a rift and their barrier spells are defined as being needed in the early parts of the game. If a hypothetical mana/stamina regenerator was included, why would they be different than needing a certain class to proceed with a mission or needing a mage to cast barrier spells in order to survive? And who'd say that you wouldn't get more options for resource healing later in the game such as the healing bombs we have in DA:I, which would prevent such a mana/stamina regenerator from being a party staple?
You make too many assumptions. We don't have definitive info on how necessary classes are. Bioware claims that you can play the game with a 4 same class setup. I assume this happens either because there are alternative paths, or because in the most extreme of cases, you can bring a mage once to fix all bridges and then you are fine. We also don't know how important the barrier spell will be. People erroneously still think that healing and tanks are important in DAO and DA2. How can you tell what is important in DAI when we don't even know all the abilities. We will have to see how this plays out.
The mmo example I gave is a way to increase difficulty in a different design. You keep the same output, but you can't do this perpetually. The question shouldn't be "why not to include" rather than "why to include". In a traditional mmo, every class has 5-6 different healing spells. The design has to provide reasons for the players to just not use the maximum output one. Limited mana is the framework, the existence of abilities that universally deal with limited mana is to balance the negative points of that decision. Which in return creates another balance problem, forcing the class that has the said ability to lack in another aspect, like healing output which then goes in a cycle and forces the designers to rebalance the output of the first class. And that's the main reason mmos release balance patches every month. There are so many parameters that simple theorycrafting can't solve, like boss fight mechanics, gear, even terrain plays a part. And I'm not even going into PvP.
Why should a few problematic abilities instead of being rebalanced to match the numbers, have to be dealt with indirectly, making the resource mechanic more than it should be? Especially when the new direction isn't necessarily resource focused (we can't choose our attributes) and classes don't have 5 different abilities that perform the same function. DAO could work with some resource tweaking for certain classes. I can hardly see how DA2 would and it's too early to say for DAI, but provided that this time around we have even less abilities to play with, I don't think it will be that different. Could be wrong though.