"Also, characters and series that have no plans for future releases, or a low possibility of future releases, inevitably will be considered lower priority."
Source
What is the ultimate determination of a series' future? Their financial success. Combined that with financial being the general biggest indicator of a series' overall popularity and notoriety, when then influences their characters' demand and profile, it's simply naive to cling to "financial success doesn't matter".
"Yeah the character's uniqueness and how well Sakurai can fit them into Smash will be the ultimate determining factor, but you can't just ignore the prior factors that will very likely keep Henry Fleming from getting to that point for that determination to be made, and prevent him from being high priority enough to beat out his competition throughout development if he makes it past Sakurai's approval and conception."
It doesn't matter how amazingly unique and mind-blowing Henry Fleming as a Smash character is if there's 10, 20, 30, etc. more characters who will get considered and prioritised first because they're more demanded/from more prolific series. Takamaru didn't make it into Smash 4 even though he was a potentially unique/interesting inclusion because Sakurai didn't deem him popular enough, and we don't have the likes of Wonder Red and Saki even though they could have been really unique additions.