It has all been done here before with plenty of comment and speculation about why the trials were limited.
What ever your view, I very much doubt any of the N reg guys are really complaining about the restrictions effecting them - after all presumably it is the CAA who hopes to benefit from the trials rather than the pilots, and in particular the N reg pilots who have probably flown plenty of GPS approaches already.
Seems to me it is in fact the CAA who are complaining about how few pilots have participated in their trial.
Incidentally I am not certain how the CAA know how many pilots could participate because aside from the comment about whether they fly and aircraft that would comply, the CAA presumably only know how many pilots their are with a current medical and instrument rating, but they dont know how many of those instrument ratings have lapsed - so presumably the number is speculation or reflects the theoretical absolute number of qualified pilots.