@thetonestarr: doesn't matter. The DSi can still have DSi only cartridges, so it can function in the same way as the GBC did to GB. Also, it does have faster processors/more onboard memory.
The DS is still selling. Next handheld will come out when sales slow, not because the DS will "have been out almost seven years". Time is immaterial when sales are high.
As for the rest-of-world market, the games likely wouldn't be out until March 2011 in other regions--a full year after HG/SS.
This is really not the implausible scenario you're treating it as at all.
Crusty VGchartz old timer who sporadically returns & posts. Let's debate nebulous shit and expand our perpectives. Or whatever.