I can at least tell you what would dispel my concerns:
1. Some kind of death-rattle clause. If your company doesn't make it or shuts down this service, all the code necessary to set up the service would be released under a permissive license, something like that.
2. The ability, even if it had a high cost ($100k a year or something crazy), to license the source to the engine that powers Firebase and deploy a modified version (obviously just for our own use).
I think those two things would mean that if I were making decisions on the technical direction of a company or large project, I wouldn't feel like Firebase might suddenly cause a major disruption in the future. At least if there were a major incident or insurmountable problem with the service, we could bring it in house and proceed without having to go back and re-engineer all the previous work.