Only thing we use from Amazon is SES (SMTP) for outgoing only. And even that we'll probably change quite soon. Too busy trying to get our big update out in January so haven't looked into that yet.
But the less we rely on third parties, the better we can debug issues.
Unfortunately, the issue with hosting (parking or even shared hosting), you share an IP address with someone else. And if that IP is shared it takes 1 bad domain, usually one caught on drops with bad history, to blacklist the entire IP.
Most issues are with spam lists and/or China GFW.
So if you host your own pages, yes it'll likely work for a while. But as soon as you catch a bad domain with bad history, you'll be facing the same issue. The more domains you have, the sooner you'll start encountering such issues.
Perhaps dedicated IPs for various customers with larger set of domains. We have 1024 IPs at our disposable and if we really wanted to, can acquire a few thousand extra. But with all of this, even setting it up, comes a price that is just likely not worth it in the long run. It's an idea but there's just 1000 other things that would take a priority to that, probably for any company. And the worst is, if a company decides to block the entire range and/or something happens where you have to change each and every IP, then what? Just sounds like a technical nightmare.
I think perhaps the itnernet eventually fully transitions to IPv6 that may be the time to start thinking about these ideas. But then blacklist providers might get smarter and start banning ranges since there will be basically a massive supply of IPV6s and I'm sure spammers will try to exploit it.