1: My "close to unity" number comes from my experience with the mailgun APIs, I assume that SES is similar.
Imagine the outrage if your national snail mail carrier were to shred 50% of all mail which didn't have a Premium Protection stamp. Why do we let the big cloud hosters get away with it?
Not if you are paying them for the service.
There are ommercial email services that aren't google/microsoft/aws. My former company used to use one (can't remember the name) that provided you with smtp servers and a set of dedicated IPs per customers that were preused by them to build a good reputation beforehand.
I think the point is not necessary that you have to selfhost email (although it can be done, some people do), but depending on a single vendor through a proprietary service/API.