Backblaze has been offering their 'unlimited data' for over a decade [1], and it hasn't ended badly for them yet.
It's fully sustainable because they only lose money on a few customers (like that one customer storing 430TB for $6/month [2]), while most customers use much less storage than that, so the service remains profitable overall.
The soft limit of its 'unlimited' comes from it being a personal-backup mirroring service and not a fully-external cloud storage, so the service only fits certain use cases.
[1] https://www.backblaze.com/blog/all-in-on-unlimited-backup/
[2] https://www.reddit.com/r/IAmA/comments/b6lbew/were_the_backb...