Well, long before when Brawl tournaments were the biggest thing, most people would've turned their heads away when it came to living off of Smash because the prize pool money offered by tournaments was not sufficient enough to live by. Not to forget that this was long before Nintendo started to sponsor tournaments and community-run events, so many of the tournaments were funded from people rather than Nintendo themselves. I believe that MLG Dallas 2010 was the biggest prize pool tournament for Brawl, offering $35,000, which was the largest prize pool money for a Smash singles event until another tournament in 2017.
Now in today's time, you can earn a fair amount of money off of Smash, but it all comes down to being signed by an organization, sponsorships, and how well you perform in tournaments. Most professional Smash players stream on Twitch, which also helps their earnings as well.