I use PostgreSQL full text search and GIN indexing and often find it to be good enough and fast enough without the hassle to have to handle a second engine just for search.
Their is a huge conflict of ingerest of giving this power to a major economical actor that vastly depends on public investment and under public scrutinity.
Executive should have the audit right and in some measure probably it should be widespread to all citizens up to sensitive data not being leaked. But what good is there to give this power solely to one of the richest and more powerful man in the world? This is crazy.
If you have an infra that need to scale so much then Postgresql isn't the right tool indeed. The right tools for your use case probably doesn't even exists and you will have to build one.
It is not a mystery why all webscale companies endup designing their own DB technology.
That being said, most of the DB in the wild are not remotely at those scale. I have seen my share of Postgresql/ElasticSearch combo to handle below TB data and just collapsing because of the overeng of administrating two DB in one app.
you can certainly encode queue in many ways. mkfifo just work. But integrating the queue in the DB isn't a bad idea if you want to have both the queue and the db in a shared state.
I am happy that my queue inherit ACID properties.
SQLite simply doesn't allow concurrent write so it is a no go for a queue.
I don't know much about SQL Server and MySQL but I wouldn't favor a lockin closed source software or anything remotely connected to Oracle.
At the end, only Postgresql remains I guess. Also, Postgresql is super solid and the closest to SQL standard.