If it was incomplete in an unsatisfying way, then sales for the game wouldn't have been as high as they are. It was obviously complete to a satisfying level for 30+ million people to buy it. Plus, they made it clear from the get-go that they would have more content coming, and they did have more (not a lot until this major 2.0, but they did have some). Also, I'm not sure anyone expected for them to include as much free content as they are in the 2.0 update (it's a LOT of stuff), and I didn't see anyone making predictions about the massive expansion planned (granted, that one costs money, unless you do NSO+ then it's free, so it will be free for me). Also, with AC, part of the joy is it having new things every so often. If it launched with everything in 2.0 at launch, it wouldn't be as special of a game. Other games you want to be 100% complete at launch, but AC is a unique title.
It was accepted as complete precisely because necessary updates were promised and delivered, but let's not forget that the game originally shipped without some major holidays (Easter, Halloween, Christmas, etc), and shops (Redd, Leif, etc). I'm not slamming the game for this, and I know it was a conscious choice, but it does mean that a cartridge in 2030 is going to be a very different, much less complete game, and I think that part is kinda shitty.