One thing nobody really says is that backblaze actually requires everything you've backed up to be accessible to the computer at least once a month, so you can't actually delete something you've backed up unless you're willing to lose it in a month, or pay extra for the 1 year retention tier. Essentially you need to own the physical drive space to store anything on backblaze, so there's not much reason to abuse it.

Wonder how difficult it would be to patch the client or just make some sort of fake FUSE filesystem to bypass this.

IIRC their client detects FUSE and friends. People have used iSCSI or ATAoE to bypass this, but that comes with it's own limitations. Most people don't bother.

I doubt $2/month is a disincentive to a hoarder whose only alternative would be to pay per GB for the storage space they are using.

That only allows them to delete things for less than a year, they would still have to develop some sort of system for redownloading all their data, putting it in a backed up drive, and leaving that drive plugged in for about 2 days for backblaze to actually notice it