I've been a member of the Ulster Flying Club for about 6 years. From when I joined until about 2 years ago there were two full time instructors (CFI + one other) plus a number of part time instructors. After the last full time instructor got a job flying jets (though thankfully he still instructs part time) the club didn't hire another full time instructor, instead the part time instructors covered the hours.
Now it looks like the club may return to having 2 full time instructors. The suggestion that 15 part time instructors will be laid off is, in my opinion, nonsense. There is no way one full time instructor could cover the hours of 15 part time. I would be surprised if any of the part time instructors are laid off.
As Nearly There points out, having another full time instructor will help with continuity of training.