But these apps aren't "proof-of-concept" apps - they are fully functioning, working cross-platform applications.
So are you saying that these companies which have implemented these apps should instead say "You know what, we have this app developed, and working on the major platforms - but let's instead devote three new teams, one each for Windows, Mac, and Linux - to re-implement this app natively! I'm sure upper management will agree!"
That's not going to work. That's going to be shot down and laughed at. No company is going to re-implement a working cross-platform application over to three separate native contexts, and maintain all three. That just won't happen. It works already. The users probably like it just fine. What benefit to the company will this get them? Nothing - just more costs for maintenance across three platforms.
If native is wanted by the users, likely what would happen - if it happens at all - will be the company says "Ok - we'll make it native for Windows, maybe even Mac - maybe. Linux? Forget it!"
It's not the developers. Developers would love to make native apps, for all the platforms. But developers are limited by the companies they work for, and by the economic realities that all the platforms can't be supported natively; at most, only one or two can - because at that point, with the number of users on those platforms, the additional costs of maintenance and support are pretty much saturated. Adding additional native platforms doesn't just add onto those costs, it actually (in theory) multiply them - because a single person might use the application across multiple platforms. So if they have problems on two or three different native platforms, now there are three different support issues (needing more people to support) - instead of a single complaint for a single platform.
I get it, though - it would be great if these apps were native, and worked on every platform, from now and into the future (even on platforms that don't exist yet!). That's just not going to happen; if native is wanted, then only the most widely used platforms will be supported, and even then, one of those will likely be dropped, and it won't be the one from Redmond.
So - what can be done? I dunno. The concept the author brings up ("use React Native") might be the solution. Or some other interface that abstracts a platform's OS and other native functionality out to an API that is the same for all platforms. But now you have issues with security and other access - which you still have with Electron, but it is more contained and constrained, since it has to go thru the Chrome engine and the various rules/settings browsers have for sandboxing bad actors. Or - you leave it to the user and their operating system (and slim it all down - maybe that's what React Native does; I don't know, I've never used it).
Or - you (that is, the company) just says "Sure - we'll do a native only implementation - for Windows only." - because that's how it usually goes.