maybe they could even use a relatively inexpensive colo/baremetal provider to simply mirror the bigtech deployment on a smaller scale (would need to be quite flexible/vendor-agnostic to make that work...)
The main problem is that would outgrow single hard drive so would need NAS. Also, the transfer speed could be an issue as database gets bigger. Even if don't store all customer data, it does make sense to store all the configuration, keys, and secrets.
If anything, it increases the need for 3-2-1 backups: the original copy of all of your files are on somebody else's computer that you have no control over. Hopefully they're keeping it backed up, and hopefully they don't go belly up and pull the plug all of a sudden. So you can use a primary backup in another cloud service from another company that hopefully won't kill their product at the same time as the other one (again, you have very little knowledge or control of the way they run their data center). Ultimately, it's a good idea to have a copy of your data that you have control over, maybe in a big drive (or set of drives, tapes, etc) in the safe, rotated daily/weekly/however long your company can cope with losing in a major SHTF situation.
Excessive? Maybe. For what it's worth my shop is locally hosted with both local and cloud backups. I have never regretted having at least one backup of anything and it's saved my bacon (or my coworkers', boss', etc.) a number of times. I've been fortunate to never need to rely on a secondary backup, but I sure wouldn't bet the company on it.
As Fred Brooks said in Mythical Man-Month: Show me your flowcharts and conceal your tables, and I shall continue to be mystified. Show me your tables, and I won’t usually need your flowcharts; they’ll be obvious.