Legend of Grimrock vastly outsold Fallout 1 & 2 combined and hit almost as much as Diablo which was simply inconceivable in those days. Second, Diablo and BG are the above mentioned "AAA blockbusters with massive appeal".
That said, I think you really can't overstate what a difference is made in pricing between games then and now. I'm not sure how big a cut retailers took in comparison to Steam (surely bigger, but not sure
how much bigger, though it's an answerable question, I'm sure). But a game that sells for $50 could sell 10% the number of copies of Grimrock and gross a higher sum. Perhaps a much higher sum, as I suspect given the number of bundles it's been in the average price of Grimrock is in the $2-$4 area. It's easy to lose sight of this -- for example, Gemini Rue has outsold most Lucas Arts adventure games, but I am extremely confident that it grossed a tiny fraction of what they did.
I'd add that I think the problem with the "sequels do worse" analysis from a data standpoint is that the data we're looking at is pretty noisy. I think it's definitely true that there was a Steam indie boom period, which has since diminished. Since at least some of these games, like Grimrock, are from that boom period, it's hard to know whether the diminished sales of their sequels relate to changes in the market as a whole rather than diminished interest in sequels. Also, bundles can so ridiculously overstate the number of copies sold that it is possible (by no means likely) that some of these games are performing similarly to their original releases if you take bundles out of the mix. There may also be title-specific factors at work. For example, Grimrock sold like gangbusters because it looked pretty and a huge percentage of purchasers (including me, actually) didn't fully realize what the gameplay was. Banner Saga was also very pretty but purchasers/backers could have been underwhelmed by the gameplay based surprise at how boring Oregon Trail style gameplay is (I thought it would be great -- one reason I backed Banner Saga was that I had long dreamed of using that same gameplay backbone) or based on the ways in which the combat has a dissatisfying feel (the way enemies are always so heavily armored, for example. Also, Grimrock did not, strictly speaking, offer "more of the same" -- it's pretty significantly different.
Since most of my data relates to my peer group of adventure games, what I'll say is that in the same period these sequels performed poorly, WEG's sales of
entirely new titles followed a similar trajectory. Gemini Rue, Primordia, and Resonance all sold >40k copies their first year based on what I can find in publicly available data; none of the subsequent WEG titles seem to have come close -- all had to be bundled to break 15k copies. It's possible that GR, P, and R were simply oodles better than A Golden Wake, Technobabylon, and Shardlight, but I'm not persuaded. I think the market just changed in some respect.
Anyway, I think VD's instinct is probably right on this, but I'm not 100% convinced.