I don't think I've ever said this on HN, but: delete your post. It's nonsense.

    Because it's so fine, you need a VERY powerful 
    fan to move any air through it, which means the 
    purifier will be very loud.

You've never used one. These Coways are known for pleasant sound and quiet operation.

"For a purifier of its capabilities, the Mighty is notably quiet" - https://www.nytimes.com/wirecutter/reviews/best-air-purifier...

Alternatively, just take a glance at the ~30K reviews on Amazon. - https://www.amazon.com/dp/B01728NLRG

    It doesn't matter how amazing your air purifier is if it 
    sounds like a tornado. If you only run it for a few minutes 
    before getting annoyed, the purifier is not doing its job. 

Top speed is about 53dB. The other speeds range from "barely noticeable" to "is this thing on?"

    Combine MERV13/14 with off-the-shelf PC case fans (which 
    have been optimized for decades to be as quiet and long-lasting 
    as possible) and the result is an easily-maintainable, quiet, 
    highly-effective purifier.

More nonsense. My oldest Coways's been in service for ~10 years including ~5 years of services running 24/7/365 near my cat's litterbox.

For a given CFM, with all other things being equal, larger fans (like the ones in Coways) are going to be easier than a bunch of smaller fans. Though, if using an array of Noctuas or similar, I'm sure it could be done.

    Many purifiers try to use sensors to determine when to 
    run the filter, but this fundamentally a flawed approach 
    and just means you have low quality air 99% of the time.

If you put the Coway on "Auto" and fart next to it, it will spin up to a higher speed for a few minutes. That seems like pretty good detection to me. You do seem to get false positives though/// sometimes it spins up for apparently no reason, so I usually just leave it on low.

    The solution to these problem is a less-fine filter. For dust and 
    pollen, something like MERV13/14 is absolutely fine.

This is certainly true. Though a lot of people buy these to help with pet allergies and want/need HEPA. My understanding is that a lot of the harmful stuff in wildfire smoke also requires HEPA-level filtration, though I might be wrong.

    off-the-shelf furnace air filters that are VERY cheap compared to 
    custom-fit HEPAs which may stop being made the moment that purifier 
    product line inevitably ceases to exist.

Once again, you're making this stuff up. Now, it's a good point in general -- nobody should buy a rando air purifier without looking into this aspect. Admittedly, standard furnace air filters do have the edge here of course. Corsi-Rosenthal style boxes are going to give you guaranteed filter availability from now until the collapse of civilization. And if you're filtering air in a large or dirty space, then yes, Corsi-Rosenthal boxes are the winners in CADR-per-dollar. I'm building one for my garage workshop before pollen season.

But have you actually taken 30 seconds to look on Amazon and noticed the dozens of third-party filters made to fit the Coways? It's like buying Bose headphones. Yeah, you'll need to replace the earpads eventually, but they've been using the same earpads for like 15 years and there's an ecosystem of cheap 3rd-party versions... not 100% ideal, but also not a problem. As you note, maybe the situation's different outside of the US.