The issue with daemonhood is that while it's the end of the 'path to glory' for a chaos champion, it's not a wholly positive thing.
For some barbarian in the old world or serf in 40k, the idea of becoming a super powerful immortal warp beast seems like a huge upgrade.
For a primarch or spacemarine captain, you already have loads of power and agency- becoming a slave to a warp power is an obvious downgrade in agency. Primarchs are basically immortal anyway, though they are not invulnerable.
All chaos primarchs got Daemon'd, some against their will, some deliberately, some out of desperation. There was never any question that they wouldn't, because that's where the dark god's wanted them.
Angron got his through a ritual of sorts on the part of Lorgar, not as a reward for service. Perturbato getting 'elevated' is not solid evidence of success- in a sense it's a failure state that puts him solidly in the pocket of the gods.
IW would have been far more interesting as a renegade legion that was totally unaligned to chaos- retaining their primarch and operating as a largely unified marine faction that fought without any of the Imperial restrictions. You could still have chaos alligned splinter warbands that play like current IW.