It's an idea, and possibly worth it for Sony to take a shot at this idea. However, as DonWii mentioned mentioned above, peripherals simply don't capture enough of the market for this ever to be more than a niche phenomenon. The reason why the PS2 continues to do well today is largely due to its gigantic installed base of more than 100 million. If new games are released that will only work with a motion-sensing controller, that install base drops down to 0, killing the biggest edge of the PS2.
Here are the groups Sony would need to convince for this to be a success:
* Convince developers to make games using the motion-sensing controls.
* Convince consumers to purchase the motion-sending controller.
* Convince consumers to continue supporting the PS2 even longer into the future with game purchases.
It looks like an attractive idea on paper: hey, why not get all those PS2 owners to upgrade with our new controller, undercutting the Wii's market! Unfortunately, the precedents for this in the gaming industry are not good. Developers rarely support peripherals; why spent a lot of money developing a game only a small portion of your install base can use? That's not a winning proposition. Getting consumers to buy an "upgrade" for their existing systems has also historically been a losing venture. Does anyone else remember Sega's disasters with Sega CD and 32X? How about Nintendo's 64DD? Consumers aren't as stupid as companies like to think they are. In the mid 90s, they could see that it was better to wait for a new Saturn/Playstation/N64 rather than wasting money on junk like the 32X. I think most consumers today are intelligent enough to do the same, investing in a brand-new Wii rather than a slowly dying PS2 with tacked-on motion controls.
Copycats almost never do as well as the original. Every succcessful game spawns imitators, but the sales rarely match that of the innovator. Think of how many copiers there have been of Pokemon, The Sims, and Doom (to name a few off the top of my head). Consumers would know who was imitating who, and I believe that the sales would reflect that.