"significant commercial application": you have no idea how many downloads or users of Ardour there are.
"mercurial set of cross-platform moving targets": precisely what is addressed.
"nontechnical users": that's a new constraint not present in the original description, but probably not much of an issue if you're discussing a specialized tool rather than a "desktop" application.
"hardware ecosystem": completely irrelevant to the case in question.
By all means, though, all the best with your startup.
The question wasn't "why it’s super rare to target Linux for desktop applications". It wasn't a question at all, in fact. It was an assertion that it's hard to package 3rd party applications for Linux, and I am asserting that this claim is incorrect.
As to whether or not the packaging situation explains why it's super rare to target Linux for desktop application, that is indeed a different question, with multiple answers.
To a product manager, this looks like a black hole where money and engineering/tech support resources get chewed up with very little value creation to show for it.
It’s also how some very mediocre distributions became enterprise standards for Linux - their value proposition basically being, “we’re not a moving target, and the Fortune 500 can buy a maintenance contract from us”. But this only happened in server land, where the hardware ecosystem isn’t so chaotic.