Yes, the sqlite concurrency model is a bad choice if you have a high degree of concurrent writes. However for many applications that simply isn't true. When it comes to websites i think people significantly overestimate the amount of concurrent writes.
Even if you don’t use message passing to use one thread to perform all updates/writes, it still performs very solidly for many use cases.
We're all using WAL mode these days, right?
The only difference between SQLite and Postgres write locking is the granularity.
But it’s talked about as if it’s a categorical limitation, the app will fail if there is concurrency. But it’s just a question of how much time will be spent locking.
A website with a 16 process pool for handling requests will be fine.