It wouldn't expand the userbase, but I do think it could help increase game sales. I think I had three GBA games when I bought a Game Boy Player, and had no intentions of buying any more, but after I got that thing, I started paying attention to the GBA again. With 3DS, I'd be more inclined to buy games sooner and new instead of used, as I often think "I'll wait until this 3DS game gets cheap since it's such a pain to play those and not worth that much to me."
I don't think a 3/DS Player is really any risk, it would be such a cheap device to make. It wouldn't need to have any of the expensive components of the 3DS, such as the screens and the cameras and all of that, it would mostly just have a cartridge slot, and maybe a CPU depending on whether or not the Wii U is a kind that could match the 3DS CPU's clock speed. Then again, I believe the 3DS has two CPUs so either way the device would probably need at least one. Still, the CPUs are one of the cheapest components of a 3DS.
I think it could be priced at $50 and still have a profit margin on it, so even if it sells similar to the low numbers of the Super Game Boy and Game Boy Player, it would be of no loss. That said, I do agree that the timing isn't right yet, there are other matters to attend to first, but in one or two years it could be a nifty device.