No reason to deep delve since rewards are not worth it...unless

By the time you can do deep delve you already have multi mirror gear and there is nothing else in the Delve itself, even on a good day, to even compete with what you might had to grind or craft to get the money for that multi mirror stuff.

And you also peaked in power, so the rest of delving it just bragging rights and just a slow and steady path to the hard limit. No surprises no fun.

But what there was something that could enable to achieve even greater heights in power nowhere else in the game can give you AND/OR you can also be useful to the rest of the market? Make REALLY deep delve start rolling to spawn Crucible Forges. Even deeper for Primeval Remnants to reach the Forge of the Titans.

Not good enough? how about Caches with LEGACY UNIQUES? starts with low lvl uniques and deeper can reach the higher lvl req ones, or maybe whatever GGG considers too broken to be obtained at the first threshold. I was about to say Recombinator rewards as well but the Settler version was made into core, so maybe some other node all the way down would be fun, specially from old forgotten league content that was "burrowed deep" and lost to time. Maybe lots of silver coins for Ancestor combat?