The scales tipped too far... the 'good's' in the balance. No outside influence and cultures become ingrown and stagnant... too much - they die. Merkel's and the European elites are wrong... they went too far.
OK, if you'll bear with the analogy: Somewhere around 1% carbon makes iron into steel. Go much over 2% and it just turns into brittle crap. Other alloy additions like chrome, vanadium, niob, etc. can be added beneficially in greater amount.
The morale of the story: be wary of who you associate with.