As Reggie has stated and I believe Iwata has implied, the next Nintendo console will be more than just HD. So the effect of not having HD depends heavily on the "more" part of the next console.
Let's see if Ican illustrate this with a current-gen example:
| Wii | No HD | HD |
| Motion | Amazing sales | Same amazing sales |
| No Motion | Sales plummet 80% | Sales plummet 60% |
With motion controls, the lack of HD has meant basically nothing as the system was sold out for two years and is still selling really well this year. You can't really beat the Wii's record-setting sales.
Without motion controls, the system would have sold poorly, with no incentive for new customers to play and almost no incentive for last-gen gamers to upgrade (VC, MIis and low pricepoint being the only advantages left). HD could offset that, but it's very hard argue that Wii would have sold more than GameCube without motion controls. My 80% and 60% figures are all conjecture, that's just to give an idea of what I think the relative impact would have been.
For the next system, HD will be more important, but if the new features and games were as compelling as motion controls turned out to be HD still wouldn't really matter*. If there are no new compelling features, the lack of HD will probably be magnified.
*This is an interesting revelation...when I started this post I figured Nintendo would suffer something like a 15% sales loss without HD even if it had compelling new features but after I wrote down my Wii example I changed my mind. I think people will continue to pay for cool new game ideas that aren't in HD.
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