Or it is main quests + the minimal amount of grind required to progress further
Yes, that's how it is. As you noticed there are some things needed to be done to progress the game further
On the other hand you could finish DAO by only doing the main quests and nothing else, so I believe it could take only 40 hours on normal difficulty.
Oh and I agree on that. Both game's main quests are similar in length, but in DA:I you waste much more of it for things like traveling from one point to another (because of larger world) or on a grinding.
So if they both are equal in length there is only one logical conclusion. There is more meneingful content in DAO main quest than in DA:I.
I base it on my logical logical conclusion to given facts.
(just my theory - it is open for pointing flaws in it of course)
[MC] - Meaningful content
==== - Travel Time.
[GG] - Grinding
DAO Start: [MC]====[MC]====[MC]====[MC]====[MC]====[MC] : 41 hours
DAI Start: [MC]======[GG]======[MC]======[MC]======[GG] : 42 hours
Now you can count During both main quests, which game had more content to it.
For me vandering miles from one point to another to get another quest ... not to my liking.
Grinding mobs Is just no-no ( as in - no JRPGS for me). My fav game of all the time VTMB has like zero mobs grinding - there is no point in doing that cause you simply not getting any XP for killing them. All of the XPs comes from finishing the quest. Great game design.