The issue i have with this idea is that it's already hard enough to find opponents who want to add a modicum of variety to their models and colour scheme and by releaseing cheap prepainted armies you'll just be tailoring to that lazy mindset. I find the painting to be the least enjoyable part of the hobby but at the same time i continue with it because having a good looking, unique force on the tabletop is incredibly rewarding for me.
What i don't want is to discover that everytime i fight space marines i'm going to be against ultramarines because people can't be bothered to come up with an interesting scheme themselves and just picked up the prepainted pack. It's bad enough trying to find someone with a unique army anyway.
Why should the people who can't be bothered to put the time in have even a half decently painted army?