They’re only a money maker if you happen to pass one above the posted limit that has cameras installed?
AFAIK mandatory reduced limits on motorways are always enforced by cameras? Not at every gantry of course, but enough to catch people.
A simple case of slow down and speed up, if you happen to be above the limit? That is what I (allegedly) do……
I suspect quite commonly people already driving in (say) a temporary 60 limit fail to spot that it has changed to a lower number, rather than intentionally speeding. A lot drive way below the posted limit when they know (or think) there are speed cameras about.