Is diffraction an increasing worry? Or, what's wrong with more megapixels?

Is that possibly because they usually don't reduce those pixel sizes until it can be done efficiently?

True, I worded that poorly. For the lenses, you only need that they not lose more sharpness when opening up then they gain from escaping diffraction. So you might see more limited benefits with a few cheap older lenses that had to be stopped down to f/8 to get sharp, but most modern lenses will be near enough to their best sharpness by f/4 or f/5.6 .

And as Tedolf points out, the real question may not be when you can no longer get more resolution, but when the difference can actually be seen by human eyes at normal viewing distances. Whether even 10 MP is enough can maybe be debated. But by the time we've got to 100 MP though, pretty much no one is going to be able to see any more resolution whether looking from 4 inches away at a 4-inch print to looking from 40 inches away at a 40-inch print.