We already have an example of what Meta would do with sideloading: trick clueless users into installing highly invasive spyware that inspects every traffic going in and out of your device, even MITM-ing TLS connections.
They did this by abusing their enterprise certificate.
As much as I'd like the ability to sideload apps, abuse by commercial vendors is a very real concern. With a few exceptions, commercial software has proven itself to be untrustworthy with the growth of surveillance capitalism. I'd rather that sideloading be reserved for free software.
https://techcrunch.com/2019/01/29/facebook-project-atlas/
They did this by abusing their enterprise certificate.
As much as I'd like the ability to sideload apps, abuse by commercial vendors is a very real concern. With a few exceptions, commercial software has proven itself to be untrustworthy with the growth of surveillance capitalism. I'd rather that sideloading be reserved for free software.