Yes, but what they're doing isn't very productive.
They're about 3500 to 4500 people.
Of course you can have a huge salary when you're in such a locked profession (irreplaceable), with such a corporatist spirit (due to being all trained at the same school in tightly knit classes of 20 guys/gals), and with a few rational arguments (money they bring, responsibility they have, etc..)
Their cause (fighting against pay worsening) might be just, yes.
But since their arguments are so specific, they're just creating a set of privileges for themselves.
Pay worsening should be fought through general strike. Not just ATC strike. The latter only consisting in allowing the random ATC to upgrade his car from audi A5 to A7...