That already exists, see Photon Engine's multiple PaaS options https://www.photonengine.com
Different ones have different APIs tuned for different types of gameplay and or Unity3D integration (by mimicking Unity's original networking programming API).
I've implemented them for an MMO I freelanced for a while back and it was pretty painless.
And actually they are running game loops, but you can't inject code into them unless you run their software on-premise, or use a separate system I recall them mentioning that offered server-side scripts that got injected in their loops.
And cost per message make perfect sense because of the networking model, which is fully message based. Paying per message is paying for bandwidth + cpu usage as a single combined rate.
And actually they are running game loops, but you can't inject code into them unless you run their software on-premise
So you are seemingly contradicting yourself. 1st you said that "they are doing that." Now you are saying they are also doing "Massively Multiplayer Online As A Service" -- but in a quite different way than what I specified. So yes, I did read what you posted. Apparently you only carelessly read mine.
You acted like no one offers a PaaS for MMOs (and gave it a ridiculous name).
I pointed out just one of many PaaS options for MMOs (or ...MMOAAAS as you insist on calling it.).
In fact, when working for that MMO we did evaluate options the did let you use their game loop with your cloud functions being called on triggers, but you'd have to look that up yourself, that was a few years ago (even Photon has added new services since I did that research).
If you think you have an approach that can enter the space, by all means go ahead, but PaaSes for MMOs have existed for ages.