Look, I hope Bioware's design decisions had little to nothing to do with keeping multiplayer in mind. But the reality of game development is they did. The gameplay is identical in MP and SP.
Tell that to tactical camera.
Opportunity cost is a fine rationalization, but there was already an opportunity cost for sustainables in the last 2 games: mana pool reserved and/or fatigue. Do you honestly think that MP wasn't the reason these were removed? And if they weren't limited to 8 hotkey bindings for real-time console controller MP accommodation, do you really think they would have limited you to 8 usable abilities? Yes, nothing can be definitively proven, but let's consider all of the evidence and make some logical deductions.
I just gave you logical deductions beginning with the premise that the 8 ability limit was not due to MP but was a part of BioWare's overall strategy with Inquisition to force the player to plan ahead for encounters. This might include an initiative to have the player think more carefully about picking their talents, which is what an ability limit does.
The claim that an ability limit waters down the game sounds dubious to me, and is probably based on gamers being resistant to change more than any meaningful loss of depth or strategy. If your mage needs to unslot a CC spell because he needs fire damage for this zone, then you might want to consider building a few CC abilities into one of your rogues. That is strategy.
Of course, it's possible that the changes actually were due to MP, but I find this claim to be based more on an inability to prove otherwise, rather than any reasoning from precedent or logic. Having said that, if BioWare does clarify in the future that it was because of MP, I won't be shocked, and I also won't feel that anything of real value was sacrificed.
Regardless of good vs. bad, it's an example of MP design affecting SP design.
And that's something I thought we were going to avoid.
The class balance already happened in DA2, so it's unrelated to MP.