Yea but Hello games believed in themself
It might not be (only) a thing of believing - it might be (also) related to other differences between those examples.
Hello Games is an indie studio and (correct me if I'm wrong) said game No Man's Sky is the
one big game for them. Not solving this games release after the hype which was there before its release ("?", as I haven't played it or followed its development in any way) might doom the entire studio. And as it is the one game, it is also clear that all forces are directed to that game, as it is this...or nothing.
Paradox is a big publisher/developer. IR is one "branch" in their GSG game universe,
among other like EU, CK, HoI, Vic or Stellaris. Its release wasn't without troubles, some effort has been made to solve problems (all the way to 2.03). If there is now for whatever deeper reasons a shortage in human ressources for development, a decision has to made which
branch/games pulls the short stick - and from Paradox economical view, I can even understand their decision (what doesn't mean that I like
it). They deciced to leave IR in basic, but solid 2.03 state and focussed on the games, which are even more sucessful and less of a risk. The alternative would have been cutting from the teams of another game, creating the disappointment and losses there (and they probably decided that both would have been higher in other GSG-branches). You can subsumarize this decision under "lacking belief in IR" of course, but the way I understand the decision any other move would have mean that Paradox would have "lost belief" in the alternatively targetted game (series).