I think it's pretty much as Redlight says. The full value riders are still around but are fewer as a proportion of the total. I reckon there are two main reasons for this:
–New riders entering who are used to the faster, sportier and more competitive pace of sportives and road clubs.
–Riders who've been around a while are becoming faster, if not in speed at least in attitude. That is, they are riding "road" bikes rather than "tourers", bouncing controls, etc.
At the same time, there's a subset of the last group who are riding faster but still finishing at "full value". Riding faster but stopping longer at controls, in pubs, etc.
Fully agreed about the thoughtless Badness of keeping Controllers hanging around unnecessarily!
There's another effect which seems a pity, with no-one at fault, and no obvious "solution" apparent:
10 years ago, at a youthful 65 years, I wasn't quick (in spite of trying hard to be!) but there were usually other riders around in the later stages of events. Now at a youthful(??) 75 years, there does seem to be (as the OP is suggesting) a decreasing number of us "laggards", and not only because I'm a bit slower still; this makes me
less enthusiastic about entering Calendar events, because watching the entire field quickly disappear up the road, before doing the whole thing solo, isn't necessarily fun; and
the fewer entries from slower riders, the more usual this gets. This in spite of the way Controllers give wonderful support to riders near the time-limit, and are glad to do so.