So no chance it's some debris or coating layer flaking off in various shapes and sizes from layer of screen or some other component inside the phone? Until and if someone tears apart a phone, retrieves that so called "dust" and microscopically and chemically examines it. Claiming it is dust is just that, unsupported claim.
There are too many inconsistencies between reports to know anything for sure. Some users have their phones in very dusty conditions and report NO "dust" inside screen. Other have dust on six different phones they keep trading in. If "dust" is entering via the silent switch as is "claimed" prove it by getting a new dust free phone, covering that switch with a piece of tape and let's see if any "dust" appears. If someone can exchange for six phones and each one has "dust" within a month. These stories sound, well to be blunt, sound a bit fishy.
If this is a design flaw as has been claimed, why after a year, and two phones, have my son and I seen not a spec of "dust" on the screens. We use them daily, in Apple case, so the silent switch is open. Carry in pocket, don't baby the phones. Seems a bit odd to me. And a design flaw like this would have millions of instances when tens of millions are sold. Not the measly few reports on this forum thread. The click bait title, making a rather bold set of statements is a strong clue this is a conclusion looking for back-up.
Extrodinary claims require Extrodinary evidence. I see the Extrodinary claim, I don't see the Extrodinary evidence to back it up.