I've horizontally scaled plenty of monoliths. Tends to be easier. Less to monitor. You don't have to worry about "what" you're scaling. You just scale it all. Just throw another server on the pile, it reduces resources used by everything on the existing servers.

If you have a process that takes up a lot of a particular resource that others don't (e.g. disk throughput), maybe spin that into its own service. But in my experience, lots of things are just fine being lumped together and don't really exhibit resource use profiles that are all that different from eachother.