It's difficult-to-impossible to truly run Postgres as a proper "herd of cattle", due to the need to failover replication roles between reader and reader-writer, ensure replication is in sync on startup before allowing reads, handle upgrades carefully, etc. If you're using something like RDS or another managed Postgres, this is still the case, it's just being handled for you by someone else.

So it's not that you're just reducing the number of hosts; you're eliminating an entire class of hosts that have complex state and maintenance requirements. Your application processes are hopefully a herd of identical cattle and can be killed off and relaunched at will, and that property remains the same after introducing Lightstream.