Imo, the 3DS userbase is getting a bit different from DS userbase. The handled market is indeed shrinking, since parents, in these times re not willing to buy a person dedicted console for their childrens. On the other end the Vita is just to damn expansive and not that much appealing to most consumers.
So in the end I think 3DS has an older userbase, has kept a portion of ex NDS users while stealing some market shares to ex PSP users.







