I don't think there will be any worry about fun of failure. Bearing in mind that it's been many years since the last expansion for GC2 was released, and since then many games have explored that concept.
FTL, for instance. I have never beaten the enemy flagship, although I've had a lot of fun trying different strategies, weapons and basically just seeing what the next run to sector 8 throws at me.
Then there's Rimworld, which at the moment sees you employing steadily more devious strategies to survive the next wave of baddies or the next crazy random event. It's fun to figure out there what you can do better next time. For instance, if I dig into a mountain way ahead of time and build a hydroponics in there or have a big stockpile of raw food to cook, I can basically wait the attackers out, being sure to barricade the entrance once I've got my colonists safely inside.
The fun part is trying these strategies and not knowing if they're going to even work.
Something that happened often enough in GC2 is that you would basically have the game in the bag and have to do a lot of tedious mop-up to get a conquest win. I wouldn't be surprised if someone's scheming up some new events to keep you on your toes in such a late-game situation. Or, dare I say it, an AI storyteller which starts those events to keep the game from getting too stale.