The Web is always in trouble for some reason or other. I remember when Microsoft came after Netscape and threatened to lock Web standards into IE. Only the Web is so big, with such reach to billions of users, that no one owns it. This means it will always be contested ground.
But the Web today faces a primal threat.
Some say the threat to the Web is “mobile”, but the Web is co-evolving with smartphones, not going away. Webviews are commonplace in apps, and no publisher of note is about to replace its primary website with a walled-garden equivalent. Nor can most websites hope to develop their own apps and convert their browser users to app-only users.