And what happens if Keikai fails long before Google gets rid of sheets? I get that Google has a habit of killing things off, but they are often things that are unpopular. Sheets is far from unpopular.
And I don't believe it's true to say they've only killed unpopular things. Anything outside of the core business (and I will argue Sheets/Docs is outside) is up for the chop.
Only one customer can do that, not all of them. As a small provider of enterprise software I would offer to sign a source code escrow agreement.
They do not have 74 million paying customers. However, I agree that the "free" version is safe for as long as the enterprise offering continues to do well (5 million PAYING users as of Feb 2019: https://cloud.google.com/blog/products/g-suite/5-million-and...).