Am I right in thinking that with each generation, it seems the console with the weaker specs tends to win? If that's the case, what does this mean for the next generation?

We'll be playing 2 1/2d games next gen?

Conicidence does not = Trend.

Yes lower speced consoles have had the adavantage but has nothing to do with the fact that they were lower speced , the average customer doesn't go "hmm which console has the least proccesing power....i'll go with that".

I benefits such as lower speced consoles being cheaper to produce therfore giving a competetive edge might be an issue , console manufacturers with lower speced consoles might focous more on the gaming experience than the hardware performance like Nintendo apear to be doing.

But to answer your question quickly , there is no real correlation between console performance and sales performance it's just coincidental if it apears to be .

If that was the case Ninty could just release a revised Wii every gen instead of R&D....

Well, let's crunch it:

3rd Generation: Master System > NES > 7800

4th Generation: Neo-Geo > SNES > Genesis > T16

5th Generation: Saturn > N64 > PSX > 3DO > Jaguar

6th Generation: Xbox > GameCube > PS2 > DC

7th Generation: PS3 > 360 > Wii

Of course, these are estimations on pure power, not on how well that power was actually utilized, otherwise a few would be flipped around. But either way, the strongest platform has never won a generation, but that's not because they were the strongest platform. The winners since the introduction of the NES have always relied on one of two traits: either their ability to "revolutionize" (NES, Wii), or their ability to attract mass 3rd party appeal (SNES, PS1, PS2)