I'm buying FP2 Day 1. I have total trust in Sakurai (and Furukawa / Nintendo's other execs) that they can come up with some really fun characters no matter what. Also, I have a history with Battle Passes - having bought Dota 2's big World Championship pass* on a nearly yearly basis.
Now I'm not saying that the backlash is going to reduce sales because I really don't have a clue. But the climate around smash DLC has definitely shifted and it COULD potentially change how many people invest in the second round of DLC.
While it has shifted,
Cutie Gwen
has a point here: Byleth's reception was not Infinite Warfare / Battlefront 2 at release / MvCI levels of PR disaster. That matters a lot in this context: yes Byleth's inclusion is divisive, but there's less reason for Nintendo to quickly respond compared to those examples I gave. Infinite Warfare is a particularily strong comparision here since Activision went "Oh crap, people are
really tiring of us going in one direction constantly" and shifted gears to more nostalgia: CoD: WW2, Black Ops 4 and the Modern Warfare remake.
If anything, my (wild) guess would be that your Average Joe / Jane isn't going to complain as loudly as the hardcore about the FE swordsmen. Generally, when things are Infinite Warfare levels there's an overwhelming consensus that things are
bad. Didn't happen here. Nintendo has enough breathing room to silently shift here. Still: I strongly doubt in all honesty that we're getting another FE rep in a long while simply because A) they have other characters to consider for (likely limited) first party slots B) and now they know of the backlash more fully.
*(Yes, love that game despite the community being really off the rails at points. That pass and its increasing amount crowdfunded for each tournament is the big reason why Fortnite held a $30 Million World Championship last year. And even that got surpassed by last year's crowdfunding.)