If you want my personal opinion, I am neutral on this, but I am open to the possibility. It's just that I'm used to being disappointed, so I set my expectations low deliberately so that any potential excitement is just that great. Some of my own counter-points to my list:
-Smash DLC is technically less work overall than a brand new game, which needs to be built from the ground up.
-Nintendo might strong-arm Sakurai into promoting certain characters like Rex, Spring Man, Waluigi, etc. if means more money, to Sakurai's chagrin.
-Nintendo could potentially exploit the popularity of guest characters (and characters in general) for profit, in order to keep the Switch userbase intrigued.
-The rights and resources aren't as difficult as we think, if we are to assume that the companies consulted for Smash are just as excited to be in the game.
The only reason these aren't looked into seriously is because it's just so conveniently good that if it doesn't happen, it'd be disastrously-bad for many. Just look at my banner. It's 15 slots exactly, just like the possible 15-character-total DLC. I haven't gotten rid of it because it contains characters I am holding out hope for. Maybe some get in, maybe one gets in, or maybe none get in. Regardless, you can show support for the appearance of certain DLC choices, but hype responsibly. Trust us, it's good for your mental health.