I am wondering, why it took so long for something like this to happen for Postgres
We were thinking about adding PostgreSQL support to Vitess several years ago. However, we realized that it would be more efficient if each tool was focused on a specific task.
High concurrency can become a problem due to the 1 process per connection architecture.
If I understand correctly what you mean, then this is no longer a problem. You will simply need to use a connection pool, such as Odyssey or PgBouncer. Even SPQR has its own pool of connections for each shard.