Far-UVC probably would have a large effect if a particular city or country installed it.
But if only a few buildings install it, then it has no effect because people just catch the bugs elsewhere.
Imagine the effect of just treating sewage from one house, and leaving all the untreated sewage from a million houses untreated in the river. There would be essentially no effect.
Yes, certain places like preschools might benefit even from an isolated install.
But that is kind of exceptional.
The world isn’t an efficient market, especially because people are kind of set in their ways and like to stick to the defaults unless there is strong social pressure to change.