complete speculation here - i have to think that valheim's initial runaway success meant their financials were very, VERY frontloaded the moment the game was available for sale. virtually everyone (or, realistically, a very, very strong majority) on PC who had an interest in survival/base building would have picked it up based on its insane word of mouth, which leaves relatively slim pickings after*
i think it's relatively safe to say that there was quite a war chest to play around with, but it's a double edged sword in that it's very very hard to continue to squeeze more sales out of the product, especially considering how slow that first year went in terms of dev with the team suddenly finding themselves with 10x the expected playerbase all reporting issues and such which delayed their initially planned content rollout. said content rollout also does relatively little to push more copies, as it's generally returning players (who may or may not drag friends along new to valheim, but generally would already have established play groups prior)
there's also probably a reasonably safe extrapolation that after a year or so of weaker sales, that income *has* to come from somewhere, and game pass seemed like a good fit, and then that's just adjacent from signing a bigger deal with Microsoft in general
all that being said though, as someone who's thrown 3 digit hours into the initial release and another 3 digits into a Mistlands run, i'm of the opinion that much like minecraft, a couple of years difference isn't probably isn't going to matter in the long run when you actually get to play valheim because as great (and as jank, lol) as it is, it very much is in the category of early access game where you get to the end of content and then end up with a long, unending wait anyway
*edit: did some digging to prove/disprove my own post:
Feb 2021 launch
Mar 2021 5M sold
June 2021 8M sold
Apr 2022 10M sold
yup, sales fell off a cliff lol
edit2: just to clarify, it's not like 10 million copies sold isn't enough to cover dev for a good, long, LONG, while; it's probably also prudent to plan a longer runway than you think you need regardless. trying to find business when things aren't looking so hot is already too late, and i think Iron Gate is at least aware that the pace of development for the game *has* been taking a lot longer than they initially thought it would
