European birth rates were/are in a death spiral. Nature abhors a vacuum. One can not expect that Europe will remain empty. People will move in, whether invited by Merkel, or uninvited.
Europe is also largely secular and faith and worship is by and large dead. In times past God has allowed barbarians to invade apostate Israel in order to accomplish his work. Europe has chosen their future.
‘In times past God has allowed barbarians to invade apostate Israel in order to accomplish his work.’
I think the aggression and ruthlessness of the barbarians had more to do with it...