Is there a way to increase the upper limit to iCloud storage space?
Before Apple One introduced 4TB iCloud storage plans, the limit was 2TB. There was no way to increase the limit past that biggest tier, and likewise now with the 4TB tier — there's no tier beyond the largest tier.
This has been a regular complaint of iCloud. The 4TB tier could have been in response to the years of asking for more than 2TB, but of course now we're in the same situation and delaying the inevitable for those of us who store a lot of data in iCloud.