I remain wholly unconvinced that side loading and user safety are even related at scale. They are orthogonal.
There are plenty of apps available through the Play store that are not safe. Even if side loading requires chain-of-trust, malicious behavior will remain rampant. I'll concede that it restricts the ease with which one can redistribute malware but by how much? It doesn't seem significant to me compared to the hassle for end users/developers.
It all seems so contrived. The only rational explanation to all of this is backpedaling into a closed garden.