Larry Graham was one of the first if not the first to popularize the Pop/Slap method. And more out of necessity than anything else. His particular style was a big contribution to the funk sound of Sly and the Family Stone. Today, there are many who can play it better than he but he was doing it in the 60's. How many players can claim that title? He was pretty much it at the time.
Not only was it just his new style of playing back then. He was also a leader in getting the best effect from pedals through a bass guitar. Not many bass players used pedals on the bass back in the 60's-70's, at least not as effectively as LG did. At that time it mostly distortion through an overdriven amp such as Grand Funk Railroad who by the way did an excellent job with bass over distortion tube used in Rock music. But it was Graham who had a rather complete pedal board for 60's-70's funk music.
It's really about understanding music history and the challenges they faced during their own time than it is about how they compare to today's players. It's not fair to compare a historic player to someone playing in today's music world.
To truly understand why some of these bass players who are regarded as legendary you first need to read their biography or life story and what they had to go through to become legendary. In most cases, they each developed or created a new style of playing that influenced many other players later on. In Larry Graham's case, he basically developed a pop and slap style of playing because the band he played in with his mother for whatever reason lost their drummer. To make up for it he played the bass with a more percussive sound to compensate for not having a drummer. This was necessary because the family needed the gigs to make money to survive. Larry is considered to be the father of the Pop and Slap style of playing. Many other bass players were influenced by his style of playing.
Larry is just one example and there are many. But to get a true understanding of their importance in the history of the electric bass guitar you must compare them with their bass playing contemporaries who were around during their particular era and not to who is hot today. There are many more talented bass players today than there were back in the day. The Electric bass is still a relatively new instrument even though it's been around since the 50's . There are many outstanding players today but they were not so common in the 70's, 60's and especially the 50's.
Every musician learned and was influenced by someone who came before them. The most common question asked during an interview of a talented musician is "What musician inspired you to play or influenced your style". They all have someone they looked up to and it is those they mention that deserve respect and a note in music history. Many blues artist including Keith Richards, Paul McArtney, Eric Clapton, Jeff Beck and Jimmy Page for example may go back as far as mentioning Robert Johnson (1920's-30's ?) when it comes to their blues influence. But more commonly, it's artists like Buddy Guy, BB King, Albert King, Elvis, Buddy Holly, Les Paul, Jimmy Hendrix, Little Richard, Jerry Lee Lewis, etc that they listened to growing up and trying to emulate.
Same sort of thing with modern bass players, James Jamerson, Jaco, Paul McArtney, Larry Graham, Bootsy Collins (James Brown), John Entwistle, Geddy Lee, on and on while some more recent, some older etc. We all have our influences, heroes and mentors.
It's about their point in time and their contemporaries rather than who they compare to in today's music. Today's musicians have a huge unfair advantage over those who simply gave it their heart and sole with almost nothing but their fingers, instrument and vocals which to make their music.
OI don't know if Larry Graham (or at least Sly and the Family Stone) is in the Rock and Roll Hall of Fame but he and they are deserving.