It's not about fun. If I wasn't already having fun with friends, I wouldn't be here. You're ignoring powerful psychological reward systems. People want to feel rewarded spending their time on an activity. When you're capped at 156 Reaper points, there is no reward because there is no progression.
This.... I have a toon with all PLs, over 156 reaper points. I have no incentive to play the toon outside of raids.
And even then, it's "figure out a build for the next challenge raid push, TR, gear, park at cap for months".
Alts help, but that's really a band aid.
I like the trees for the first 100-120 points. It gives you options on how you wanna spend them up to the point where you've maxed out the tank tree + melee or caster. But after you've already farmed the 156 to max out all 3 trees - there should be some form of progression. It doesn't have to be huge. But if there's no sense of accomplishment - you won't do it.
I've done R10s from harbor to cap, i've done challenge raids (pushing difficulty and/or lowering group size). You can say I've done almost everything you could for bragging rights.
There are no bragging rights for having 300 reaper points, since it's still the same as 156: you maxed out your bonuses. Even if it's a small linear progression of 5hp>2 melee/ranged power>5 universal SP>10 spellpoints on repeat - that's already so much incentive to keep pushing my RXP.
One doesn't have to have RP rewards go beyond 156, they could hardlock it and do a new system. GW2 introduced a rewards track system with various objects that reset daily, weekly, quarterly, and with festivals/etc. where new rewards are added quarterly and old stuff still left available to get too (Think of it like HC10 where at 5k you could claim any of prior 5k rewards or the current season's reward). Imagine of all the rewards SSG could do under a system like this:
Quarterly LEADERBOARDS!
Cool cosmetic gear
wings/footprints/mounts
X amount of RXP
Instant past life of your choice
new sentient jewel voices
X amount of sent XP
X amount regular XP
etc. etc. etc.
It would give reasons to log in daily, weekly, etc. for various folks. And by leaving old rewards available still, it removes FOMO issues too. Think of all the various objectives they could do:
Kill X of Y mobs.
Do X quests. Do X quests on Y difficulty.
Do X raids. Do X raids on Y difficulty.
PvP and win X times... (Could you imagine? hahah)
TR x times. ER y Times.
Earn X RXP or Y RP.
Do Y objectives on a 1st lifer character with no tomes used and without access to shared bank.
Etc. etc.
Gotta be careful with "dailies". Because at some point it just turns the game into a chore of "gotta log in every day to do the dailies, or i'll fall behind". It's tough to balance between it becoming a must vs not worth bothering with.