Can the decline be stopped? Yes, but that would require a great unlearning of the political mythologies on which modern Europe was built.
Among those mythologies: that the European Union is the result of a postwar moral commitment to peace; that Christianity is of merely historical importance to European identity; that there’s no such thing as a military solution; that one’s country isn’t worth fighting for; that honor is atavistic and tolerance is the supreme value. People who believe in nothing, including themselves, will ultimately submit to anything.
Europe decided that Christianity, the historic foundation of their system of education and moral order, was worth only their disdain and derision. It's been replaced with nihilism. As Hemingway used to ask in the midst of artillery shelling, "How do you like it now, gentlemen?"