I’ll try to keep this short.
I think every key passive should be uncapped rather than capped to maintain balance between classes. Yes, before season 9, certain classes had uncapped key passives that allowed them to scale beyond other classes, but Blizz should have gone the other way. Uncapping every key passive will give players goals while tempering and masterworking, as well as through paragon. When you cap everything, you remove all variety and variance. Instead, perhaps give a loss of return when reaching a certain breaking point, and if players want to go beyond that, ALLOW them to make that decision. Capping = removing fun. Why remove the fun?
Because it results in easier planning for seasonal powers, aspects, and uniques, as well as less bugs in future patches (id say less testing and easier balance, but the dev team doesnt tarry with such nonsense)
tldr: the things you dont like about caps, the devs do.
With all the limitations they’re introducing and the prescribed top powerlevels that leads to, they might as well just introduce sets.
Huh. Not only key passives, the whole game is going there.
I’ve been staunchly against sets for quite some time, believing that the concept behind aspects gave much more variety in buildcrafting.
Given how that is part of a system of completely unbalanced nonsense that, along with the difficulty scaling changes means you either play meta and destroy content, play competent builds but off-meta and struggle, or play anything below that and you might as well not play the game at all, sure, add sets, maybe then the devs can have actual diversity with some inkling of balance.
Probably not, but maybe.
What you suggesting is softcap instead of hard cap the passive. Softcap is going to be mathematically complex to implement as it needs a formula with diminishing return, this will lead to bugs or under performance of the passive if not tuned correctly.
The use of softcap actually make it more complex for average players to plan their build. For example you have 10 paragon points which you can allocate to passive already reach soft cap. You need to calculate whether is it worth to continue put the points to the passive with severe diminishing return, or use the points on main stat which has small multiplicative damage. This same as masterworking, players have to decide whether to masterwork an affix which already reach softcap or use it on other affix like Attack Speed.