It's just that we don't see any convincing arguments yet to embark on creating something from scratch when the main problem (as far as I can see) is mainly manufacturing output.
Yes.
The train of thought for them is not a
complete fallacy, though. It has a few realistic, even remotely possible elements to hold the otherwise unrealistic story.
There apparently are some short periods when there is a lack of ventilator equipment in some hardly hit areas, because it takes time to increase ventilator manufacturing. This is true. Now, the story goes like this: some super bright team of science nerds in their garage hack together an iffy-looking, but good-enough
thing, build a dozen of them, bring them to a hospital, and save a few dozen lives. (Add some dramatic music here.)
This is like any fantasy, it has remote chances of happening, but the fallacy part is that every "open hardware" idiot is writing themselves in the lead role of that story. What they don't realize at all is that starting to build a bureaucratic community, from scratch, with all the broken telephone play with people they don't know on the interwebz, has already failed before it even started.
They almost always realize they can't do it, or even a meaningful part of it. They don't realize that due to this, they are not needed, and that no one who
could do it, is unwilling to participate to their ego projects.
If this fantasy were going to happen, it would be with a few
actually bright guys and gals,
who already freaking know what to do, to just start doing it, like immediately. No "project planning" needed.
Open source or not, is a
completely irrelevant aspect, as are the details whether the design uses Arduino or 3D printing, or not. Anyone who gets fixated to such details are automatically proven morons who can't understand much beyond buzzword and popularity contests. While important to some, they don't get the job done.