When they start rate limiting and banning your users they'll be the ones demanding refunds from you.
On top of that users would need to commit their data to Google's ownership and if they're using the free versions of Sheets good luck if their account gets locked because the algorithm demands it to be so.
Take your pick of lockout horror stories:
Edit: Also don't forget their Maps API change that sent many developers to the wall https://imgur.com/gallery/ZuWMooq
But yeah, the comments below about rate limits & bans? I was probably wrong to try to advocate Sheets, however much technical sense it made, however good a working experience would have been.
A company I used to work for had a Google Sheet with finance data that contained a script to scrape it from an API we ran - totally legit. Just a trigger that runs a script that pulls in the data. Google deemed this unacceptable (we never found out why) and locked the entire sheet.
We tried to get in touch but never succeeded. We nearly lost all the data in that Google Sheet, but thankfully someone had done a "Save as Copy" a few days earlier.