Patent trolls (the most successful ones, anyhow) practice Batsian Mimicry as a defensive strategy: they specifically try to look as much like a small inventor who's invention has been stolen by one or more bad actors. It is never simple to disrupt an evolutionary arms race.
> I imagine a simple addition to patent laws invalidating a patent if the holder does not commercialize it after a certain period should get rid of NPEs. Is it not that simple?
Are you aware of how long it takes drugs, for example, to get from the patent stage to the commercialization stage? I'm sure there are other obvious industries where this would be problematic to the normal way of doing business.
On a more broad note: is it good to require that all inventors directly commercialize their own inventions? At best, I think this is an open question.