Do you also act as an SRE for those apps hosted on your bespoke hosting platform?
Before, we had many situations where they asked me to take a look because something could have been a software issue but then turned out to be an AWS issue instead. That's always nasty if I need to tell my client "sorry you're offline, try calling Amazon". Even if they understand that there's nothing I can do, emotionally they still feel disappointed in me. When I use my own infrastructure, I can avoid these situations because it's in my power to make the necessary hosting repairs, too.
At their spending level, "Megacorporation X" would not pick up the phone for them. But I do. So in effect, my competitive advantage is not that I'm not a megacorp, but my competitive advantage is that I pick up the phone when they call.