That's a key question: pay Linode $1,560/mo for a 3-node 32GB RAM cluster or launch 3 32GB boxes for $720, figure out PostgreSQL replication, make sure you setup the replication users, make sure you don't open any security holes, make sure you have Patroni or Stolon so that you can switch over when the primary fails, make sure you have etcd or Consul or something to handle that coordination, make sure that you have Barman or pgBackRest setup to take your WAL and persist it to S3, setup your S3 buckets, setup a full backup schedule so that you can restore easily, make sure that you're regularly testing your restores, make sure that you're testing your failover (do you even know if Patroni is actually working?), figure out how your app is going to cut over to the new primary when that happens (is there a shared IP that you need to move, are you using a proxy like HAProxy that's checking health-checks to see which it should proxy to, etc).

Little bit off topic: It's funny that all the time you see comments on HN saying something along the line: why using Kubernetes, it's just overly complex. But after that are, most likely, returning to their self managed DB clusters that need all that maintenance and setup just listed. While, if they were using Kubernetes, they would have Operators or Helm Charts that do 90% of the stuff.

(Now back to topic) Don't get me wrong, I'll chose a managed DB all day every day over something self managed but sometimes (e.g. startup without a indefinitely amount of VC money) self management of infrastructure is the fastest and cheapest way to go.