If Canonical are too controlling though then they’ll get criticized for censoring / being over-controlling etc. Atm they’re taking a very radical neutral stance when it comes to managing the store. I think there were plans for some sort of verified publisher system though? Also I think the warning message you suggested could work.
Again it’s up to the users to work out what works and what doesn’t. Ubuntu reviews (at least) have been disabled for snaps and Flatpaks (in GNOME Software), maybe these need to be re-implemented on snappy’s level (rather than GNOME Software/Ubuntu’s level) to allow for independent assessment of the quality of software. Outdated and unmaintained snaps would run the risk of getting one-star ratings and reviews, which should be useful for the end-user…
Some in the community already despise certain snappy design decisions for being over-centralizing so Canonical going further here would potentially incur more anger, rather than less.
However you can see, for example, this decision for a precedent for removing outdated/unmaintained apps, as you can see, the store managers were very reluctant to do so but there is a precedent, at least, for removing an outdated snap when the snapcrafter isn’t contactable and when there’s an existing snap which is up-to-date.