The underlying problem generally with Air Traffic staffing is you have a small number of qualified people doing the job.
Say you need 4 or 5 ATCOs to run a radar bank or tower, to add another for contingency adds 20/25% to cost base. The sums are very different for an airline with 100s of pilots qualified on the same equipment.
True, and not true as well. Especially considering that having a generous standby level at all times is cash wise peanuts considering the rest of the cost of running ATC at the worlds busies single runway airport. And even more so considering not doing so will increase the pressure to legislate financial responsibility in for a service promised and not provided, a service airlines have to pay handsomely for. After all, they are liable for measures they have to take to accommodate their customers. And for exactly that reason airlines nowadays actually play for peaks, not averages, and usually a higher than normally needed crewing level. You can operate a shorthaul aircraft on a full schedule at 3,5 to 4 crews per aircraft. However, it is usually done at 6 to 7 as that allows a much higher stability of the operation. Which is needed, in part due to ATC systems all over europe having not the required personnel and no financial incentive (liability) to provide it.