The trolling gets in the way...
When I see "proprietary software" I think branded closed-source products where access to the code is either impossible or requires payment of significant fees.
I don't see anything Canonical is doing re: Snap that fits that. Their Snap Store itself is an irrelevancy, as all such shops are. They're user-side programs to access a repo. "Store" is only a metaphor.
Hostile reaction to all-things Snap sure looks like kneejerk trolling intended simply to punish Canonical for not adhering to someone's demand that everyone in open source must abide by the notions in their head. Or maybe they're still mad about Gnome or whatever.
Snaps, flatpak, appimages and the technology surrounding them merit serious investigation, experiments, and testing. The trolling just makes that more difficult.
It is obvious that all the commercial Linux players are moving toward containerized/immutable/etc systems. It makes good sense for those kinds of markets. And, they're competitors in those markets so, of course, they're fielding different approaches to the same end in hopes of locking down market share.
For individual Linux users and enthusiasts, I don't think it will make much difference as long as debs, rpms, and other traditional package systems are supported. I suspect they will be supported for a very long time because Canonical, Red Hat, etc., will spend years, if not decades, trying to transition their existing paying customers.
For me, if I can tweak and fiddle with things as I currently do, I'm not sure I care about snaps vs flatpaks vs appimages. That's not the case, now, however. There are applications I use that I would not use if I couldn't do use an external tool to adjust one thing thing or another that the app itself does not expose to users. And that's why I don't use flatpaks or appimages.