Given that norms say nothing about it doesn't mean they are always but the possibility exists that the extremists really are too emotional and partisan, and getting what they wanted would be the proverbial dog catching the car and not knowing what to do with it or crashing it immediately.
The "center" people are referring to here is a United States-centric form of liberalism. It's not any less "extreme" than communism, it's just the dominant political force and, as you say, has the privilege of positioning itself as the comfortable default. In reality, it's a fairly extreme commitment to a different set of principles.
Commoditization is just a fundamental result of math basically. Fundamentally there is still a cost to a commodities no matter what party pays for it to provide it or bar access through certain methods as illegitimate like it being illegal to buy organs from a donor for transplant.
Centrism is de-facto the status quo. Which, in current times, means support for the police as it exists, supports for business regulation as it exists, supports greenhouse gases and military conduct as it exists, supports the behavior and election processes of politicians as they exist, etc.
Its simply the privilege of the majority to be the "moderate" ones. To be radical is not to be good or bad, it just to be outside the norm. And it makes sense that those in the norm find those outside it dangerous and threatening, because... they want to be. Thats the whole point. Their time is wasted if the goal is not to become the new moderate, the new normal, the new center.