I think that's a bit misleading because iTunes and Amazon also sell DRM-free music, which I don't know the legal implications of, but at least there are no ridiculous technical problems here.

Also, if the App Store _sold_ software, that still wouldn't mean that the upgrade process would necessarily be flawless. (OTOH, then we could all just use Sparkle again, which has worked without a hitch for me since 10.4.)