Not every PC gamer is in the potential target audience of Fallout 76. That doesn't increase the numerator -- it dramatically decreases the denominator, making those 2 million a much bigger percentage.
Either way, I feel vindicated with Fallout 76 now being the lowest rated Fallout game in history, selling >80% fewer copies than Fallout 4, etc. Hopefully Bethesda learns a lesson from this and starts putting in the same effort as their peers in the industry.
> their insistence on sticking with their old creation engine
...because it is a common thing to say about Bethesda. But the reality is that they can stick with their engine just fine - they just need to fix their bugs. The problem isn't that they keep using their engine, everyone does (as you already wrote in the first paragraph here), the problem is that they do not give their programmers the necessary time to fix the issues with their engine.
> they won't invest properly in updating it for modern times
This is very different from "moving away" from their old engine and isn't just a matter of language precision: one requires rewriting everything from scratch in a multiyear project that is a big undertaking on its own (with results that are very likely to not be as stable and bug free as people would imagine) that practically no company with an existing codebase does anymore (especially at the stage and size of Bethesda), whereas the other requires fixing a few subsystems that are already there and a tiny fraction of their existing codebase that can be done as part of their updates between games. It is certainly a much smaller change than the renderer updates they've done so far.
About the Steam hardware survey, every PC gamer has Steam installed (there might be a few who do not, but they either only play one or two games, like Battlefield or somesuch, so they do not really matter as far as PC audience comes, or they are so few that they are statistically insignificant - for all intents and purposes Steam has a monopoly on the market and both developers and publishers use every single statistic it provides as pretty much The Truth). With that in mind, PC buyers for Fallout 76 - or any other AAA game - are inside this audience, either as a whole or as a subset, so this 2 million is actually the best case. In practice it is most likely lower because not every single one of those 2m gamers would be interested in Fallout 76 (it isn't like having an ultrawide monitor is some sort of requirement to like the game :-P).