He was absolutely nothing like Steph is today. He was good in a fringe sport, that doesn't make you a "superstar" any more than, say, the best lacrosse player is an American "superstar" today. Sure he might be the best in his sport, but if there was real competition it would look different.
Hell, when he starting playing the NBA was only in its 5th year, and he struggled to even get onto a roster:
* Refused to report to his first team because he was trying to start a driving school. He demanded $10k to play and they only offered him $6k.
* Didn't get to play for his second team because they folded before he even joined
* Eventually made it to a third team, which didn't even want him, and got a $9k salary
"The Tri-Cities Blackhawks drafted Cousy, but the point guard was unenthusiastic about his new employer. Cousy was trying to establish a driving school in Worcester, Massachusetts and did not want to relocate to the Midwestern triangle of the three small towns of Moline, Rock Island and Davenport. As compensation for having to give up his driving school, Cousy demanded a salary of $10,000 from Blackhawks owner Ben Kerner. When Kerner offered him only $6,000, Cousy refused to report. Cousy was then picked up by the Chicago Stags, but when they folded, league Commissioner Maurice Podoloff declared three Stags available for a dispersal draft: team scoring leader Max Zaslofsky, Andy Phillip and Cousy. Celtics owner Walter A. Brown was one of the three club bosses invited. He later made it clear that he was hoping for Zaslofsky, would have tolerated Phillip, and did not want Cousy. When the Celtics drew Cousy, Brown confessed: "I could have fallen to the floor." Brown reluctantly gave him a $9,000 salary."
That's the sort of Mickey Mouse league the NBA was in the 1950s. NBA salary cap for a team was $55k, which was about half what Joe DiMaggio was making all by himself.
Cousy's salary eventually peaked at $26k in 1959-60 (by comparison, the average public schoolteacher salary was $5k, similar to what Cousy was first offered as a rookie). Compared that to Mickey Mantle, who made $70k that year and $100k a couple years later. Or Willie Mays, who made $80k that year and eventually $165k. Basketball just wasn't as big a sport and the "stars" weren't as big of stars. If basketball had been more popular in the 1940s and 1950s, and if black folk had gotten fair opportunity to play, and if some of the athletic talent in football, boxing, and baseball had gone to basketball instead, Cousy never would have won any MVP or been considered the top point guard of his era.