I'm sure if ISIS or Al-Quaeda wants to send terrorists here...
...they'll come in on a jet plane, with valid European passports and tourist or student visas, and probably first-class tickets. Screening may be OK for catching criminals or fugitives, but real terrorists will have perfect cover.
Now, I don't doubt that maybe some guys will decide to become terrorists AFTER they have been here for a while, but there's no way you can vet them out ahead of time.
There are very good reasons to control or restrict entry into the country, but stopping terrorism just isn't one of them.