This isn't a slippery slope: this is an analogy. Your example of settings makes no sense as you have failed to draw a similar analogy and your example of bookmarks betrayal your own understanding of the system you like as bookmarks are in fact accessible via the omnibar :/. All of the things in my example were of a similar form, and frankly it would be convenient if anything you wanted to get rid of all went down the same hole: if you actually could solve that problem well, it arguably would be good UX.

The only reason the recycling examples seem absurd is because 1) it simply isn't that difficult to remember "trash goes in a black in and recycling goes in a blue bin and mail goes in... a different blue bin ;P" and 2) it is obvious how hard it is to separate the two results in the general case and the ramifications seem silly to tolerate given the low effort that the user has to take on to prevent it.

The exact same thing is true of the omnibar: there are a bunch of heuristics that often work well enough and people who are particularly lazy and don't want to learn the difference between the various bins don't have to because we figure it out for them... most of the time. I personally think that people shouldn't be so lazy, but it seems like the entire goal of "UX design" is to empower lazy people to "not have to think" (which is very different from "intuitive": after all, "the only intuitive design is the nipple; after that, everything is learned").

The reason then I think it is "destructive" goes further, though, than "people who are lazy do not deserve our sympathy": it is because, as someone else pointed out, there is a cost to leaking a user's intent to the search engine when they wanted to type in an address (in the same way that mailing someone's trash out randomly might be a privacy violation). Google doesn't give two shits about this: they want all of your information. That Google is training people to conflate the process of searching for something with then process of navigating to something is great for them and sucks for Firefox as Firefox kind of has to follow or be screwed and yet it seems a bit antithetical to their mission (though I don't think they even know what their mission is anymore).

(FWIW, the true original sin is the address bar itself: the address bar is a status display of where you are... making it editable is arguably sort of "intuitive"--using the typical kind of "just so" stories that allow people to claim random things are intuitive because the story was flowery enough--but frankly it just makes no sense and causes all kinds of problems. If you fix this, and return to the paradigm that some of the earliest browsers have, I think some of the other things the omnibar does are less awkward, and the "out" for true navigation wouldn't be affected as navigation would feel entirely unlike searching: it would become intuitive that these are separate concepts.)

(Also: the fact that I am unhealthy is not because I don't want to know how to be healthy or because I am not prioritizing understanding it... it is because most of the time I can't for the life of me figure it out even in consultation with doctors, and when I do know things--such as that I need to walk more every day--I don't prioritize doing it because I injured myself a couple years ago and I am now somewhat depressed over it. I point this out because it is very different from not knowing or not caring to know: a life un-analyzed and without curiosity at every level of scale and in every interaction would seem to me to be a life wasted. If you truly don't know anything about how a car works, I recommend spending some time to learn: it's fun! Tim isn't like the mechanic ever figured out what was wrong with my car until I finally pulled the error code of the ECU myself and rubbed it in his face. Learn to play at least one musical instrument of every class so that you can extrapolate the rest. Study linguistics as early and as often as you can in life so you can at least appreciate the beauty of other languages even if you don't choose to become fluent. Avoid hiring people to do things for you until you know how to judge if they did a good job, and decide how you wish things to be done before letting other people tell you what they like and then don't back down!)