Well, I'll take back what I said about the majority. I do still think that the remaining 1/3 of users who don't have enough storage to turn on iCloud Photos qualify as what lapcat was asking for, though.
I know users who would prefer not to trust Apple for anything, and only pay for and use iCloud to backup the Desktop [and similar locations]. If they were to hear that their opt-in for iCloud means that Apple starts copying random things, they would not be happy.
[OT, I use Arq. But admit that iCloud is simpler, and it is not apples to apples.]
IMO, the fact that Apple backs up your keychain to the Mothership; and that this is a "default" behavior that will re-enable itself when shut off, reflects an attitude that makes me very distrustful of Apple.
Large set? Yes. Majority? No. CIRP says 2/3 of US Apple users pay for iCloud storage[0]. It's this popular for the exact reason you mentioned. Almost no one can fit their photo library into 5GB so they opt in to the cheap 50GB for $0.99/month. 50GB is enough for a lot of people.
[0] https://wccftech.com/paid-icloud-subscription-is-apples-most...