PoE sold over a million copies along with a successful KS campaign. 2, however, has a much higher budget, only a fraction of which was handled by Fig. The two situations aren't very similar.
One million copies or not, PoE1 was very divisive in my opinion. Yet, it still attracted enough people for PoE2's figstarter. Hence, my main point is that even if Deadfire slumps in terms of sales, Obsidian just needs enough people to figstart PoE3. Obsidian does not need to use sales to attract a AAA publisher, it just needs to
trick convince people to donate to fig for PoE3.
Edit: I am not calling it sound business strategy, but kickstarters and fig are funded by random nerds and not businessmen.