There are valid reasons to use v4.8 (Win 9x anyone?).
QFT, and as I said, I don't want to begrudge legit users at all....but I would say Vegas odds put a lot of these people sitting on 4.8 either running pirated Windows, so they can't/wont upgrade their SP to be compatible, or using a cracked license.
Besides that, nagging users is not necessarily a good way to convince users of updating. Most users of old versions would know already about the existence of new ones. Nagging usually pisses people off, whether is a computer program or any type of "publicity" or any type of nagging (even from a real human ). If Avast wants to get people to update, nagging is not going to get the objective achieved.
Well, conversely....what happens when they boot up one day and find they will get no more definition updates? This has the potential to inflame all on its own. They likely push the nag to get as many of their legit users upgraded as possible to minimize the fallout. And there will be fallout, as Avast! is one of, if not the last, with such old compatibility, people will feel "abandoned", whatever the good reasons for doing so.
Also, they have to push this upgrade warning so everyone sees it....a lot of people boot up their PC all day, but spend maybe an hour of that day actually sitting there....these people need frequent notifications, they could easily miss anything. There is no way to cater this to the whole user base, it is quite literally the lesser of the two evils.