Hi Martin,
Like you, I used to offer hosting and maintenance for my clients' sites. As WordPress evolved, it became a real hassle to stay on top of all the security fixes and performance requirements.
Eventually, I migrated my clients to a Managed WordPress host and shut down my dedicated servers.
One of the arguments I've heard most often is that it's generally not a good idea to sell your clients an ongoing maintenance solution with a single point of failure... you. If you've got a team of developers who can help you with this, rock on. But it's generally better for your clients to connect them directly with a reputable host.
Or, you can partner with a Managed WordPress host like WP Engine or Flywheel, who offer white-labeled hosting. They take care of the servers, you take care of your clients, and if/when you need tech assistance, you've got a whole team in your corner.
These days, it's hard to beat the performance and security provided by the top Managed WordPress hosts.
Before I migrated my personal site to Nexcess (then Liquid Web), my homepage took more than 5 seconds to load (screenshot). After migrating the site to Nexcess, I was astonished the same page loaded a whopping 4 seconds faster. (screenshot)
So, I recommend either partnering with a major WordPress host to handle the performance and security issues, or simply refer your clients to them directly. Most WordPress developers I know today opt for that route and stay out of hosting and maintenance.
Just another opinion. Hope this helps!