Two things with your post
Firstly, the idea of 'more options does not = more depth'. You're going to have to do a better job with explaining that more options actually is counterproductive. You gesture towards a lot of options with a few known best options, which would mean the extra options don't actually increase the depth, they don't add anything, they're just there, anyone coming into the situation would have a false perception of depth, because really there's limited options, there's only 10 good ones. That doesn't mean that more options = less depth though. If those other guns were set to be situationally different, then there would be cases of them being better, at least in circumstance than the other guns. More options doesn't = 'things added that don't make a difference because they're inferior in every way'
Take Smash and rock paper scissors. The number of ways to approach someone on stage or off stage you can say is just like rock paper scissors. "Well, things tend to be countered by things, which are countered by other things" While mix-up and changing abilities to be unpredictable is no doubt important, options = depth is transparent here.
If currently in smash 4 the number of ways to approach rock paper scissors + 1, or a total of 4 options, then that's 4 options people are going to be learning to play at a high level. With more 'depth' it'd be rock paper scissors + x. How many options are enough? I don't know. There is such a a thing as too many. But you can't argue that more options of approach, say 10, wouldn't add more depth to the game without saying something else about it, because that's not how depth works. Sure, people would take longer to learn that or the ceiling could be set so high that the game would be labeled as 'absurd' but more options = more depth.
Rock paper scissors = 3 things, simplicity
Rock paper scissors + factors or other things = more depth, that's just how it works
Secondly, the options would have to be quantified as 'meaningless' to fit the criteria of shallow depth, false depth or meaningless not-really depth. When people are saying they want more options, they want more options, not an option which is objectively worse in every way than the other few top counterparts.
Even if something isn't used often it can be used occasionally as a circumstantial ability which can add to depth. As example; Marth's down air is not the same as his forward air, so even if his down air is bad in every category, from reaction, to mind games, to prediction, to recovery, to speed, compared to his forward air, if his down air was different or behaved differently, there are a few, even if it's only one situation, where Marth's down air is good, either because it's good in that situation, or because it's just generally good for that one certain thing.
The thing that has always impressed me with players was their speed. It's a combination of accuracy, prediction, reaction decision making and mind games. Games have more or less of that depending on how it's made. I like options, I like depth. There doesn't need to be an astronomical mechanical requirement to play adequately, but I still like options. I like complexity so everything isn't so simple , I enjoy nice flow and play. That's transparent in a lot of games. The meta changing is a nice thing, the meta stagnating due to a lack of options or depth, while also being boring to watch or play is the worst outcome for any game. A game should be really fun to play and watch. Nothing is for everybody, but games can put their best foot forward.