OK lack of relevancy isn't a valid reason? Well fine then what do you have against him I would like to listen to your long list of things that strengthen my argument XD
Lack of recency is rarely a reason for any character to be unlikely. And recency isn't always a reason FOR a character either. Not appearing recently does not shun you from Nintendo's History, and old characters are brought back in video games all the time. In games like Mario, Zelda, and Donkey Kong, they bring back characters people thought were dead and they often become standards again.
Shadow Mario however is overshadowed (no pun intended) by several other Mario Doppelgangers and I can bet you he won't appear unless MAYBE in a Mario Sports title (just because they like to exploit Bowser Jr.'s paintbrush). But, he's no worse than Sheik in terms of relevance, in fact he's more recent than Sheik and appeared in 2 Mario games where as Sheik only appeared in 1 Zelda game.
Not to mention I'm TRYING to strengthen your argument. I'm just saying you are weakening your argument by using Shadow Mario's recency as your reason.
My reason is mostly because it is incredibly weird to have a different version of a character playable within a completely different character. Like if Poke'mon Trainer had a Raichu with it's own moves. And that it's incredibly weird to have someone that's basically a dark version of someone else but, they of all characters are not semi-clones. Another situation is having Dark Samus not transform into a Dark Zero Suit Samus, because it doesn't exist. And Dark Pit having his own moves or even being a separate character when he's CLEARLY just a recolor of Pit.
And Bowser Jr. has shown he is more independent and utilizes SEVERAL of his own moves and only used Shadow Mario as a disguise to fool people. If you want to utilize the paintbrush than make it his Upward Special. It's Bowser Jr's thing, not Shadow Mario's.
And I'm too lame to own a GIANT ROBOT CAAAAAAAR.....