What about custom installs?
Your suggestion could prove more trouble than it's worth.
Adding code with each update and each version, Free, Pro, AIS and Premier,
could be a daunting task indeed. Not to mention avast becoming bloatware.
No, keep it as is. It isn't that difficult to download the program and the uninstaller.
In fact I prefer it as it is. Before I do a clean install I always download the uninstaller.
What about those who want to just uninstall without re-installing?
Some users do this for testing purposes. Then at a later date re-install avast.