There are a multitude of reasons why LeBron is worth a max salary.
It's nice to think of the fact that Surgeons, Firefighters, those sort of people should be the highest paid people in the world. And in theory, yes, they provide the most critical service to the public of anyone.
BUT... you don't get 20,000 people paying an average of $50 per ticket to come watch someone do open heart surgery. You don't have people walking around in a pair of Nike Surgeons. You don't see people wearing surgical masks and gowns in the street.
You don't have the media covering every single second of a surgeon's life. Breaking down every move they make. Creating constant tv shows, news articles and the like on surgeons. One surgeon doesn't provide employment for a whole host of people to analyse that one surgeon.
LeBron is widely regarded as the best at basketball in the whole word. Something that millions upon millions of people do every week. There are a lot of surgeons in the world, and the very best probably get paid good money too (maybe not $20 mil, but there'd be some on $1 mil I'd imagine). Then as the tiers go down, people get paid less. Same deal with basketball.
LeBron took reduced money for 4 years to play for the Miami Heat. He was the undisputed best player for a team that made the NBA Finals every year of those 4 years. They won the Championship 2 of those years. He was Finals MVP 2 of those years. He was League MVP 2 of those years (and twice previously too).
If anyone is worth a maximum contract, it is LeBron James. Maybe he will take a reduced contract again, but he shouldn't be expected to. Hell, Eric Bledsoe of all people is requesting one. There's talk of Klay Thompson asking for one after his rookie contract expires. Yes, the EBA means that LeBron will earn more for his max contract than those 2, but it's still the maximum amount of money they can earn.
There's never been an issue before with any other legitimate superstar getting paid his worth. Kobe accepted one and he's 6 years older than LeBron and has been constantly injured the last 2 years and his team has been mediocre at best.
He moved to Miami to win Championships. He won 2 and was close to winning another 2. He took reduced money for 4 years, in the middle of his athletic prime for the good of the team. He didn't completely get the results he wanted for that investment. So now he's asking for what he's due. And good luck to him. Any team in the league wouldn't blink at offering him $20 mil if they had the cap. Not one team.
There's a salary cap in the league. An agreed amount of money that teams are to spend. So, that money is going to be spent one way or another. It's not like him taking less money means that less money will be spent. So him taking more money isn't affecting the common man, him taking more money is just affecting another multi-millionaire in the league who might otherwise get to line their pockets with extra money.