How confident are you in saying that? I know that "HHPD type" effects are actually observable in, for instance, children of elementary school age. Optical illusions worked on me, at least at that age, and what child hasn't rubbed his or her eyes until they see a shimmering fractal curtain of phosphenes at least once (against their mother's wishes)?
It's very likely they occured before just as frequently as they do in the present time, you're just more aware of it now thanks to the visual distortion induced by 'shrooms. You may try asking some friends of yours and doing an informal experiment, see if people can see "halos" around lights if they concentrate at them, or see patterns or other strange stuff on a flat white surface (especially after "priming" the visual cortex/retina by staring at something displaying say, a simple motion animation.)
THe grey/static in dark areas has a name, it's
eigengrau. The current hypothesis is that it is not always totally black because your retina always has some cells that sense light that get triggered by random thermal noise. The visual process in humans is actually really complex, what your brain "sees" is not direct input from your eyes at all, like one might expect. It is instead a mental model, which is continually updated with input from your eyes, ears, memory and so on. I think it is foolish to assume that our biological intermperation of reality is a perfect and flawless image anyway!
This is all kind of pushing away the unpleasant answers to your questions. Because as far as we know, all the underlying effects of HPPD are present (if you look for them) in most sober people, they are just trained to ignore them better. The most effective treatment for HPPD is learning to train your brain to basically edit the "errors" out again, rebuilding trust in the accuracy of your vision, by conditioning yourself to ignore the disortions and concentrate insstead on things that matter to your every day life: images and objects instead.
You will eventually learn to not be so freaked out by it. It's important to remember it's not a sign of damage - if anything it is a sign your visual cortex is tuned for maximum sensitivity. Distortions should not interfere with your ability to read or judge distance, so it is a good idea to have your eyes tested entirely. You'd ideally want to test visual acuity, optical pressure, and corneal tomography if possible. If you can rule out anything physical in your eyes, consider that you have been blessed with seeing that which normally remains unseen.
The main thing is to not worry about it. One treatment that was suggested is clonazepam, an anti-anxirty drug. It does nothing to combat the visual distortions, it make you not care about them though. And not care about a lot of other things. So in the end, training yourself to ignore the distractions naturally is the way to do it.
