When I say "sustainable", I don't just mean that the company might not have enough money to keep its lights on
It's more that some product manager or VP will notice this free service and want to attach their strings to it.
Having worked in big tech, it happens very often. Google Groups is a famous example at Google (among many). It was way better 15 years ago, but it just wasn't "worth it" to keep the lights on in its current state, so it shuffled through a series of product managers/teams who wanted to "improve" it.
Microsoft has a long history of this as well. I feel like this is another generational thing -- people forgot what Microsoft used to be like, and how their free products turned hostile. So far Github is OK, but I'm sure glad there are alternatives.
I prefer to just pay money to a company that provides a service, and I hope they profit from that. That incentives should be aligned. So far with Dreamhost that has worked out exactly as expected.
If I had spent any time migrating to the latest and greatest thing in the last 13 years, that would have easily burned the money I have saved by using a free service.
e.g. I have hosted sites on other platforms that are now down and useless, as I'm sure many others here have.