It’s possible Nintendo wants to reward players with a quick unlock each time they relaunch the software. It’s not entirely clear if this is an exploit which may be patched or if it’s intentional. After doing this, the “quit and relaunch” technique started working again. The fix was to unlock one character through a more intentional method - in our case by completing a round of Classic Mode, which triggered a new challenger. After around 50 characters, this technique stopped working for us. Rinse and repeat and you can unlock all characters much faster.
The fastest way is to start a 1 life stock match, and simply run off the edge. Ultimate (press X while it’s selected) you will get a new Challenger after just one match. But if you return to the Switch Home screen and close Super Smash Bros. Normally there is around a 10 minute cooldown before another Challenger approaches. Play the fight and hopefully you’ve just unlocked your first character! These are random, so the order is different for everyone. Play a couple of Smash matches or through one character’s Classic Mode until you are triggered with a “Challenger Approaching” screen. Fastest way to unlock all fighters in Super Smash Bros.
There are several ways to do this, but there is a little trick that brings the total playtime required down from around 10-12 hours to approximately 90 minutes. Ultimate has a massive 74 fighters in its roster, and you need to unlock 66 of them.