You can sideload on Chromebooks if you enable developer mode. But enabling developer mode wipes all data on the system and displays a warning message on boot.
So Chromebooks only let you run software that a Google employee approved, unless you wipe your device to enable developer mode (which nobody, after using it for a while and then deciding to develop something, will want to do)? There is no other way to run some test code on it? I didn't know that and it seems strange (even Android is more liberal than that, and that's not a laptop) so genuine question.
I think wiping a Chromebook isn't as problematic as wiping another computer. The OS settings and apps can fully sync, and almost all apps autosync all data to the cloud.