I think we have to wait and see how much momentum gem.coop can build. Right now they have promised "things for the future"; they will most likely also deliver eventually. But right now they are not there.
If and when they open beta, though, I'll begin to republish my old gems (not all, some I merged into other gems but most of the core stuff will be back) there. They have some things they should improve on though - documentation (also a problem that ruby doc was separate by the way), namespacing (this is in part also a problem that ruby had no primary way of namespacing; this is also a feature, but it should have a way to separate concerns when possible or wanted).
Anyway, I think we'll soon see what happens - I say people should evaluate again in about half a year or so, say like ... end of May 2026. I think this would be a more realistic time frame.
I do, however had, also suspect that DHH may become the biggest asset to gem.coop - every further snide remark he does on his blog, will gain new people who are upset, and some of those will eventually help contribute and benefit gem.coop. So for the end user this may be a win-win situation since they can install things how they like it, thus having more flexibility. Many can and will stay with rubygems.org, others may prefer gem.coop, many others will probably use and combine both (this may be a bit more difficult; guess gem.coop needs to think of a way to specify different gem sources on a per-gem basis too. Lots of work to be had for certain).