I get the point your making but unfortunately you're completely wrong on all technical details described:
1/ The downloads folder wasn't an OS convention that browsers adopted. It was actually the reverse. Some browsers created that folder and defaulted there and then Windows later adopted it to it's portfolio of configurable user directories (like the Pictures folder). I was a heavy Windows developer at that time and had been working on my own 3rd party browser so I remember that transition well.
2/ Also there is no such convention on Linux nor the BSDs. Some desktop environments such as KDE will copy the conventions used by Windows but KDE has always been influenced by Windows's classic desktop shell (explorer.exe) so it's little surprise that KDE will come with a Downloads directory. However there are other desktop environments and standalone window managers won't create a downloads directory and thus browsers are back to creating one themselves. Sure you can argue that Linux is fragmented so doesn't count but the same is true for BSD and all the desktop "hobby" OS's I've used too (aside ReactOS - but again it aims to mimic Windows). So this isn't a universal cross platform convention as you described but really more of something that just Windows (and possibly OS X?) plus a few mobile OSs have adopted. However if your sample size just consists of browsers on Windows then I'm sure "Downloads" would feel like a standard to you.
3/ This is also limited to mostly just the big desktop browsers (Firefox, Chrome, etc). Many smaller browsers - and particularly ones not based on Chromium / Blink - don't follow that convention. So again, if you're only comparing Chrome, Firefox and possible IE and/or Edge then I could see how you might consider this a standard but the reality is that's not the case.
When you consider that there are as many - if not more - exceptions as there are instances that follow that convention, it really becomes hard to even call this a de facto standard; it's certainly not universal across all platforms (browsers nor desktops) like you argued it was and it wasn't even a convention which Microsoft created as an OS standard which browsers were forced into adopting.
This is why I keep pushing back when you say "standard". It isn't. It's not standardised and it's not even de facto.
4/ You argue it's the responsibility of the OS to educate people where stuff gets downloaded to but that's just passing the buck. Yes Windows (for example) could make stuff more discoverable for sure but it's the browser which the users are using to download their actual files on so that's the browsers UI they're interfacing with and the UX they're experiencing when the confusion hits. You wouldn't blame the oven for cooking a bad meal if someone misread the instructions in a cook book. Blaming the OS for a bad UX in the browser is equally misguided.
However I do agree with you that some users are afraid of their computer but then you are just reiterating the argument I opened this tangent with.