"Sideloading" refers to data transfer between two local, peer devices. Really, that is it. It is not "something scary" or something forbidden. It is not even really installing. It's data transfer.

So "before walled-gardens" people would install software in many many ways. I originally typed it in from scratch, or from a magazine. I loaded it from tape. Or diskette. That's not really "sideloading" if you think about it, because it's just "loading" from peripheral storage.

Later, when people dialed up on a PC, they could "download" software and then install it or do whatever with other data or media. They could also upload it. They could transfer it among devices locally. This was not, at the time, called "sideloading" but just transfer, or "null modem", or "sneakernet", or "a station wagon full of backup tapes".

If we're going to use "sideloading" in the strictest sense, then we cannot actually refer to the process of downloading APK files separately and then installing them, because that's literally downloading. But that is the colloquial meaning now.

Hey, if you want to coin a new term or neologism for it, by all means do so. But it seems absurd to downplay "sideloading" as having "scary" or "negative" connotations, when it really doesn't. You've got to look past the hype and F.U.D.

Remember, there was a time when people considered FTP and torrenting to be dangerous or subversive. Perhaps they still do.