Many routes have very thin yields and a modest increase in airport charges can make routes, that were profitable, unprofitable overnight.
Quite true, but that's not what you actually said.
As was proved with Shannon, a €10 tax (passed on to passengers) could be the difference between whether an airline makes a profit or loss on a route.
I don't believe that the big batch of routes operated by FR from SNN were ever profitable; rather, that the travel tax was a convenient excuse to cover the potential embarrassment of a failed experiment.