With the general push (at least in Europe) among regulators to decouple (see investigations around app stores, bundled default apps, payment services on stores), I wonder if this model will fall apart some day. The "free" iCloud and Google storage tiers seem to have stagnated, whereas the cost of storage falls.
While you can't do everything supported by first-party iCloud storage (specifically iCloud backups for one example), the files app and support for storage by API ought to provide some competition for these overpriced first party storage options. If regulators force unbundling, we could see a separate ecosystem of storage from apps/services.
Maybe this would help drive more transparency of the true costs of the products and services in question, by decoupling them from "pay us 20x market rate for more storage, since you can't use any other storage with our app".