I don't think Nintendo will have the advantage. Let's use the DS as an example. There are more 3rd party core titles on the DS vs the Wii, meaning that those titles sell decently. Meaning that there are more core players on that system vs the wii (but the core players are probably at a younger age vs core players on the PSP).
The 3DS will be at least as successful as the NDS, but the bottleneck there is the casuals that already own the older system. The question is how willing will the casual audience be to upgrade to newer hardware? You can also apply this question to the wii where the casual audience is bigger than the core audience (you can figure this out by looking at which games are selling on the system).
Backward compatibilty will help, but then again look at the PS3, Sony had to remove it to cut costs, because of something like needing to keep some old hardware (CPU/GPU) in the system. Hopefully they'll figure something out by making it multi-core or whatever.
