You mean like CloudKit?
Apple SDKs exist and do things - including everything Steamworks does and quite a bit more.
If Apple decided to only allow apps distributed through them to use their SDKs or services, then would it would be fine because they'd be like Steam?
I don't recall ever playing for cloud storage on Steam, though.
Steam Cloud is truly a backup service. It's not fast even for tiny amounts of data. They'll even kick you over to an even slower lane if you store anything over 250 MB.
Meanwhile, you can do near real-time app synchronization over iCloud between devices.
But yeah, it'd be great if Apple bumped up the free tier size. That said, I've never actually had any problems storing app data on iCloud. Apple users seem to either pay for more storage or not backup to iCloud, so from a developer perspective, eh.
Apple is perfectly entitled to ask for a 30% fee, as long as they allow for competition on equal footing (for clarity, this means they don't try to collect exorbitant rent from their competitors first). Let the free market sort it out.