Fallout and Divinity series are exceptions.
I don't think studio size or production costs or target audiences have much to do with whether sequels should be pursued. It's a question of delivering a vision, and if that means more iterations and second helpings then that is the best route.
Original vs sequel:
Legend of Grimrock: 936,949 vs 246,684
Blackguards: 471,616 vs 178,528
XCOM: 3,304,215 vs 823,999
Shadowrun: 723,457 vs 613,408 vs 188,034 (arguably Hong Kong was the best iteration but few people cared at this point)
The Banner Saga: 592,139 vs 43,826
Success of the first game often fools developers into thinking that they can do even better or at least as good with a second 'bigger and better' game, but it's rarely the case. The only exceptions to the rule are games that offer building, sandbox, and well-executed killing loop activities that people never seem to be tired of. Darkest Dungeon is a fucking monster but I bet if they go for a sequel it will sell less than a third of the original.
Vault Dweller's point makes sense if and only if a smaller-medium studio creates a product that is only mildly successful - enough to pay the production costs and keep the lights on for a new game- and those developers have lots of other ideas on the table to pursue. In other words, it works for Iron Tower Studios.
Most indie studios create products that either fail financially (either sells fuck all or sells well enough for an indie like 50-100k but still fails to cover the development costs or barely breaks even like Legends of Eisenwald) or sells just enough to continue.
A couple random thoughts on this.
First, it may be premature to judge. A lot of these games saw their total sales spike after significant discounts / bundles / etc. So it may be worth checking back in a year or so to see if these numbers even mout more (
cf. Shadowrun).
Second, I think a part of the problem is that runaway indie* successes often reflect a hunger for novelty coupled with groupthink and word of mouth.
(* Here perhaps "indie" should mean something that includes major but atypical products like XCOM.) I think Grimrock is a great example of that. A hard core of fans who actually like that horrible (IMHO) kind of gameplay had been starved of it for a while and loved the game. The visuals were pretty and new(ish) -- a genuine improvement on a classic model. Suddenly it's getting a whirlwind of good press and enthusiasm, it goes steeply on sale, and tons of people buy it for $10. Then they play it and discover that they don't like the stupid Dungeon Master two-step, they don't like RPGs with no plot or characterization, whatever. Thus when the sequel comes out, even if it's better than the original, everything is lost: the thrill of the long-deprived fans, the innocent (ignorant) enthusiasm of people unfamiliar with the genre, the excitement of great visuals from an indie team. There's nothing left.
I think the same is true to some degree of Banner Saga. It may not have been meeting a long-felt need, but it had these cool new visuals, clever new gameplay concepts, etc. But the heart of it was a lot of grinding and maybe some other structural weaknesses -- a lack of exploration, for example, fairly shallow character interactions and limited tactical variations on maps (at least in my experience). Again, you have people rushing to it en masse, playing it and realizing they don't really like it, etc. For example, Banner Saga's achievements indicate that fewer than half of the players make it to the Denglr godstone, which as I recall is someone in the first hour or two of the game. Only 15% completed the game on normal difficulty. I'm sure some will try the sequel to see if it's fixed the problems they had with the first, but maybe not.
I played the first Shadowrun for a bit, and I think it falls into the Grimrock scenario at least to some degree (pretty graphics, but the gameplay isn't quite as good as you think it'll be), but that kind of game has a broader base. I can't speak to the others, as I didn't play them, but I assume a big aspect of XCOM was the scratch-an-itch. I know I spent years and years longing for a new X-Com of decent quality, though by the time Xenonauts came out I had lost my interest. Alas.
I'd like to think that AoD is a different model, a sustainable one, just as WEG's point and clicks have been*: it's not really offering some crazy novelty or mass enticement. (* Excluding Shardlight, which seems to have been undercut by discounts on other WEG titles.) More or less the people who buy it know what they're getting and will probably like it, and they'll probably come back for sequels / successors because it's a formula they like.
[EDIT: It appears everyone made these points earlier in the thread. Ho hum]