This list of all FRC teams ever might help your analysis
http://www.team358.org/files/frc_rec...Teams_Ever.xls
It'll tell you how many numbers were never assigned at all. There are 663 non-team holes.
Also, be aware that a number of teams have possessed multiple team numbers, so those defunct team numbers do not represent lost teams.
The pre-number teams are included in that list above along with later reincarnations, so they are counted too.
We're at 45% school district FRC participation on Long Island and we've lost about 11% of past district FRC teams, although some still come & go.
That also doesn't mean the school districts lost to FRC aren't doing FLL or FTC/Vex or some other STEM robotics program still.