I still think at least 80% of the content should be made for a casual audience, even make that 85-90%, however the casuals themselves will realize that their efforts have no value when they have the time to do harder stuff with the gear they have acquired.
D3 actually had a great eternal realm playerbase until the paragon levels got insane for some players that the rest couldn’t compete. D4 has a huge advantage here because of the 300 paragon cap, just something like a greater rift end game system with non seasonal leaderboards give your eternal realm chars a 100% more value because they currently have 0% value.
Also PoE2 early access already has a ladder/leaderboard for charactor progress, so you can understand why the hardcore players are preferring it already.
Also don’t underestimate the value of hardcore players that help others in different ways, whether to be advise or doing stuff with others ingame. World of warcraft player numbers actually collapsed when hardcore players had less reason / motivation to play.
Running guilds in classic , tbc and half of WOTLK was actually worth it to achieve something. This brought communities together. When the incentive was no longer there the game numbers collapsed.
This is why PoE2 numbers remain consistant a long time after early access. The hardcore players are the glue that keeps communities going.
D4 could end up being easy come easy go if they don’t add some progress content.