1 is a scalable, managed, highly available service, with economies of scale the other is a fixed size, capital expenditure with fixed performance, limited DR, requiring a couple of SRE/DevOps and colo
There is also the will it always work question
Eventually all programs will be able to read email.
Plus size is only one limit, you would be limited to 1 write every few milliseconds. My napkin maths estimate is that there are at least 1-2m writes per hour going into this thing, so probably 300-600 writes / second (Average) and maybe over 1k writes/second peak. We are going to fall over here!
Not sure why some people seem to have a viwe of "There is no scaling problem that can't be solved with a sufficient enough number of SQLite databases".
https://sqlite.org/lang_attach.html
'Transactions involving multiple attached databases are atomic, assuming that the main database is not ":memory:" and the journal_mode is not WAL. If the main database is ":memory:" or if the journal_mode is WAL, then transactions continue to be atomic within each individual database file. But if the host computer crashes in the middle of a COMMIT where two or more database files are updated, some of those files might get the changes where others might not.'
Most of the novel work in LedgerStore is probably around managing the headaches of distributed storage, not the persistence layer.