No.
In a sense, the Ps3 and X360 need each other.
The Wii has boomed and made a massive name. That would have happened even if the Ps3 was SD (or well, assuming otherwise would make everything ridiculous).
To penetrate that immense name, the competition needs a lot of good games. You don't need as many just to hold it.
Anyway. The 360 has a lot of its games simply because they were originally (when people thought Ps3 would be dominant) planned for the Ps3 (not saying they were built for Ps3... You get the point, right?).
So, the 360 would have far less software, and the Wii would have had more. This would just continue snowballing (due to 360 falling behind in userbase) and could ultimately lead to an early death.
The Ps3 and X360 need each other. One being SD would hurt the other immensely. (It wouldn't necessarily have hurt the SD one though).