Running your server on someone else's hardware isn't the same thing as using not using open source?
AWS's servers themselves run on an Amazon-modified flavor of Linux. I'm pretty sure this version already is a fork of CentOS or RHEL.
If you choose to use AWS, you can choose a variety of Linux flavors to run.
If you choose to leave AWS and you have to find a new hosting provider or need to procure hardware to host it yourself, that has nothing to do with the provider being open source or not. Them forking their versions of Linux really only affects Amazon internally, they're not giving their internally used version out to everyone for use. They have Amazon's Linux 2 which they do give away to everyone to use, but why would you use it when there's more open versions of Linux available?
Once again, this seems mostly like people confusing using open source software and using hardware that someone else owns. Open source isn't about who owns the hardware, that's a private property issue. That's more akin to setting up your business on Amazon's lawn and then getting frustrated when Amazon isn't mowing their lawn and your business can't be seen from the road. Honestly, that's what you get for setting up shop on someone else's property where they already have their own shop.