Not sure. On one hand, the walled garden approach does seem to have some benefits when done well, of drawing in further resources/development.
> Indirectly consumed by OSS
Yeah, that could definitely be the case sometimes too. When OSS Communities use offerings present in these walled gardens (eg Affinity Designer, a good vector graphics app for OSX) to further what they're doing.
Not sure if parasite is correct :), but it seems reasonable there's some interplay between the produce of the walled garden(s) and the external ecosystem.
From the OSS Communities perspective, I'd kind of expect (but am not really sure) that many of the resources used in the walled garden apps come from OSS. eg things like gzip, various other libraries, perhaps graphics (etc).
And as a note, I don't personally believe all proprietary software is oh-so-evil. ;) The companies which develop software choose to act however they do, evil or not. The "evil" is in the actions of the companies, regardless of the OSS-or-not nature. That's just my understanding of things thus far anyway. :D
I think you missed the part where a vital piece of the ecosystem, namely proprietary/non-free software, no longer exists.
Which resources are then indirectly consumed by OSS (at least its "free as in beer" variant), acting as a parasite in your analogy :) The money has to ultimately come from somewhere, you know, or the music stops.
What would software devs do if all the oh-so-evil proprietary software disappeared? Exchange FOSS PRs with each other, until they die of hunger?