> Why not just use the right tool for the job ? (i.e. hosted DB or compute).

Because sometimes you want a database for 10000 rows, or you have 5 logins a month and you don't want a $30/month database running in the cloud.

There's a market out there for real "server-less" database that charges you per rows stored, is priced per read/write operations, and is reasonably priced for a 10000 rows per month, without having to calculate how many 4KB blocks you are going to read or write.