It's an interesting point you make, and I'm not sure I have the answer. However, I have some relevant data:
Roughly 18,000 students graduated from FRC last year plus ~5,000 from FTC (from
here and assuming equal distribution throughout the grades). That's 23,000 FIRST students graduating each year.*
According to
this site, 3.3 million students graduated high school in 2013. Combined, the two numbers tell me that .69% of all applicants have participated in FIRST in high school. That's not a lot.
Based solely on that, I would say that FIRST has not become pedestrian. However, I do realize that many FIRST students are applying to the same schools (MIT, Stanford, etc.). I'm not sure where you got your 50% from, but that seems unrealistically high except maybe for MIT.
If you want to go to those top-level schools, though, you can't set yourself apart on only one thing--you have to have many strengths and successes across the board. Take a look outside the lab--there's a world of awesome stuff out there.
*I'm not counting FLL because it's not as impressive, nor are you doing it as you apply to colleges (unless you're a genius, in which case there are different things to be writing about).