My main problem is that not only do I feel it was unnecessary - you don't need multiplayer to sell if you build a game right (Witcher) or strike the needs of your core audience (Skyrim). You just do what you do well.
I also feel that a lot of the "simplification" to put it kindly, just happens to play nicely with the added multiplayer mode. I'd gladly sacrifice it at an altar if it meant getting my attribute choices, full weapon and armor abilities, ability bar, weapon swap, inventory in combat, tactical change in combat, auto-attack and tactics back.
If it makes the game better for a large chunk of the population, and makes the game more profitable, then it pretty much IS necessary. EA is, in fact, under obligation to maximize shareholder value. 
Now if it ends up making the game worse, then it's a problem, but it would seem the devs are confident it doesn't (and it didn't for ME3).
That said, I don't agree that any of those things "play nicely" with multiplayer mode, and indeed some of them STILL exist in DAI even though they're only useful in single player, which does rather undermine your argument.
- Attribute Choice. No impact on multiplayer. Being able to choose a new ability or a new ability AND attributes makes no difference whatsoever, whether it be in SP or MP. Either way you need to choose and assign something.
- Full weapon and armour abilities. Hum, not sure I follow here. In depth crafting is in both SP and MP.
- Ability Bar. MP is not a real factor in this limitation, there's plenty of MP games with tons of abilities. The 8 ability restriction is meant to require player choice and "builds" at more than just the skill tree level. It's a _very_ common design element found in many games out there, and has no real relation to multiplayer.
- Weapon Swap. We didn't have that in DA2, a single player game. Meanwhile, ESO _HAS_ weapon swap, an MMO. Clearly no relation with MP.
- Inventory in combat. The decision to restrict inventory in combat is obviously a design decision to prevent players from carting around 2942 weapons and swapping them all the time: by preventing mid-combat inventory changes, it forces yet another tactical choice on the player (I'm going to be fighting enemies vulnerable to fire, and others to lightning... which enemy do I want to prepare for more?).
- Tactical change in combat. I think that's a point in favour of the devs actively coding in features only used in DAI. How come there's a Tactical Cam mode that is only used in single player, when the devs "simplified" everything for multiplayer?
- Auto-attack. How is this an MP simplication? For one thing, there IS autoattack in single player (in tactical cam mode), and there's no reason why multiplayer couldn't have auto-attack. It was just clearly a design decision to require a button press to attack (maybe to make action mode more visceral, or some such), and certainly is not some sort of limitation imposed by MP.
- Tactics. Hum, there is still a Tactics and Behavior menu in DAI, which is entirely useless in MP (as you're always controlling 1 character). This is yet another system that was coded in specifically for single player, undermining the claim that MP simplified SP.
MP isn't an evil boogeyman that corrupted our pure single player. Devs made design decisions you don't agree with, that's fine, but don't blame MP for it.