The US isn’t falling to these groups. They can however cause major unrest like they are doing now by mixing in with legitimate protest movements and causing issues, rioting, and violence in cities controlled by
You are discounting their ability to grow and develop. They have already done that to some extent over the past two weeks. They don't have the organization of the IRA, but that is somewhat by design. If you study Antifa they are decentralized on purpose, but have writings about organizing and how they function. The cells have leadership and they cooperate. I'm in no way saying the antifa flag will fly above the Whitehouse. What I am saying is they are a reasonably organized group of cells that can grow. They are loud and they are violent. They also have people in power sympathetic to the cultural Marxism cause. The try to use violence and the threat of violence to get their way. They and others on the left are also doing all they can to silence any voice of dissent. They DO NOT want free speech and freedom of thought to continue. They want to control the narrative in public and private through intimidation. That is what this cancel culture is. This may all fizzle out, but it can also grow. These people are dangerous. No one thought the things that happened in Germany, Russia, China, or Cuba could happen there either. They are no where close to that right now, but the Nazi's, Russian Communists, and Maoists were all a small minority as well in the beginning. When you are complacent that's when you lose your freedoms.
What has been happening in our colleges and universities has now spilled out into the wider culture as it was destined to do. These people that are radical leftists are going to be in positions of power. Look at the outsized power a socialist like AOC has or the employees of the NY Times that got the head of the editorial board fired for simply printing the op. ed. of a sitting US Senator.