There are other things to consider. For example, new DS buyers. I'm in a position at the moment where I can quite comfortably buy a DS, like it was a packet of peanuts. However, I haven't bought one yet, as I'm rather confused over this whole "DSi" situation. Do I get the DS Lite, or the DSi? Do I want to be able to play my GBA games on my DS but alienate myself from any exclusive DSi features in games, or potential whole games altogether? Or do I want access to new features/games, but not be able to play my GBA collection on my DS?
If there was no DSi, I'd probably have a DS already. So Nintendo have technically 'lost' a sale. No big deal for them, I suppose. The second thing is that the DSi may steal some of the thunder from the DS' eventual true-successor. The DS2 may appear less-amazing to those with a DSi. Admittedly, these are arguably minor things, but they add up. The DS is not in the same place that the Game Boy was when it recieved a feature-upgrade like this. The Game Boy had been around for 9 years.
Ultimately the DSi does create some confusion. I think they (or we) would be better off if they just let the DS run as is it for another few years, and then release a proper successor. There's no grey area, then. It's not even like the 360 situation. The Elite 360 still does everything the Arcade 360 does. The DSi loses features the DS Lite has, in order to make room for the upgrades. This is a confusing situation.