As I understand it, snap the package format is not proprietary. Its as open source as say flatpak. What is proprietary is Canonical official snap store, and they patch their version of snap to only use that store. It'd be the same as flatpak being tied to only flathub.
Of course that goes against the spirit of FOSS, but there's a bit more nuance there than simply saying "snaps are proprietary".
Snaps don't just suck from an ideological but also practical perspective, as described for Thunderbird. Firefox on Ubuntu has also serious permission issues with webcam support OOTB even experts are struggling with (involving AppArmor, pipewire, snap, and FF device config). and has become unusable for things like browser-only MS Teams on mainstream notebooks.
Containers, popular as they may be on servers, can only add breakage and overhead to desktops, especially for an established and already much better organized system like Debian's apt. There just haven't been any new desktop apps for way over a decade that would warrant yet another level of indirection.
I've tried making snap packages, but I discovered they're very tightly tied to Ubuntu's base packages. They're not portable at all. In essence they're effectively just a secondary Ubuntu-specific package format for user-level applications.
For example, with flatpak you select a base runtime for your package that contains mostly system-agnostic libraries. With snap, you specify an Ubuntu version as a base runtime and additional dependencies that are Ubuntu packages.
My understanding is that the base layer (similar to what FlatPak provides) is shared and downloaded by the snap manager so it is portable as long as you want to download it.
The end result should be similar to FlatPak where you have practically no dependencies as it should package almost everything.
See, that's the issue. I want my distribution to distribute the dependencies I need to run applications outside of containers. That's, like, it's main job man.
Snapd still hardcodes Canonical's snap store signing key and provides no mechanism to add your own keys. Any other snap repos will be treated as second class citizens.
Of course that goes against the spirit of FOSS, but there's a bit more nuance there than simply saying "snaps are proprietary".