What agreement? They offered a service with no guarantees that it would be provided in perpetuity. If they offered "free unlimited storage for life" and then backed out of that based on some legalese on page 75 of their EULA then I agree that would be slimy. But that is not what happened here.
> they are able to momentarily drive their prices down, eat the losses long enough to run their competition out of business
This can happen and when it does it is anti-competitive. But that is not what happened in this case. Do you really think that Google thought they could drive AWS and Microsoft out of business with below cost storage on Google Drive? Seems unlikely. And even if they somehow managed to do that, they would immediately have competitors undercutting them on price again as soon as they raised prices enough.
What seems much more likely to me is they offered unlimited storage as a competitive feature of Google Workspace thinking it would still be profitable even with some customers using much more storage. But because we can't have nice things a bunch of people realized they could essentially use the unlimited plan as an ultra-cheap object storage service and were storing 100s of TB of data. Rather than raise everyone's prices to subsidize bad actors, it seems much more reasonable to just discontinue unlimited storage.