Terror attacks are nothing new in Europe, it is, admittedly, in a form that is rather new, but then again, one cannot expect something like this to stay the same.
ANd Europe doesn't have to have an airtight grip on ISIS to be safe, the terror attacks are sporadic, most of them are not even linked to ISIS (except people claiming they do it in name of, which is bad in and on intself, but really there is no securing against lone wolfs like that - they would've found another excuse to kill people, this time it was ISIS). Yes what Breedlove said is very worrying indeed, but the fact the EU has identified 1500 ISIS fighters says something about the security services in Europe, be it individually. And really, going off the claims of a single 'ISIS operative' to take a wild guess at how many ISIS fighters are in Europe is letting yourself fall prey to fear, which is exactly what the goal of terrorists is.
They will not by a long shot even come remotely close to destabalising Europe by the rate at which they're going through us, not even remotely close.
Not to downplay the impact terrorist attacks have on individual ives - they're horrible and ruin families - they are insignificant losses when compared to the death toll caused by smoking, car accidents or people falling down stairs.
If I were to claim car accidents will bring down the stability of Europe and make a serious argument about it, I'd be put in a mental hospital. But somehow, we think terrorists will conquer the lands of Europe and have of force-choked on the qu'ran in a matter of years...
And while ISIS fighters posed a refugees on 13/11, they were still French. THe Brussels attacks were performed by Belgiums... Nice attacks were performed by a Tunesian who had been living in France since 2005 (6 years before the Syrian civil war started)
Granted the German 2016 attacks were performed by someone who entered as a refugee, but that is it for the larger terror attacks. The rest were so small scaled and only perpetrated in name of ISIS rather than the terrorists themselves being likely to actually have been part of ISIS that I still cannot see the point of how enormous a threat ISIS is to the EU.
Yes, they must be fought. Yes, major resources should be made available to counter-terrorism. But no, it is not a significant stability threat to European countries.