If you have many app servers and they all run copies of the same app you can roll out new versions to a few servers at a time. You just have to handle the db up

If you have many app servers and they all run copies of the same app you can roll out new versions to a few servers at a time. You just have to handle the db update first but you need to do that with microservices anyway (they might use smaller databases and therefore making it somewhat easier).