This is pretty absurd to even write given he was the one that steered them away from that. After Satanic Majesties he was the one that said they need to go back to basics.
His talent and vision not only out stripped the others by a big margin, but the songs we all know would not exist without him. His marimba on Under My Thumb makes the song, it was literally dead in the water before that, and the Sitar on Paint It Black is absolute perfection. The recorder on Ruby Tuesday (a song which he actually wrote), the dulcimer on Lady Jane, the harpiscord on Play With Fire. The list is insane without even getting into his guitar work, like the riff for The Last Time or the slide on Little Red Rooster.
He was wildly troubled, as genius tends to be, but his contributions can't be undersold. Can't say if it would been any different if Oldham hadn't frozen him out and bullied him, but regardless his influence even lived on well past him in the Stones. Even at the end he was impacting beyond his playing, the most important instrument in Gimme Shelter is the guiro, it's what takes the song up to the level of greatest rock song of all time, the atmosphere of violence and fear that song instills, and that is directly from Brian Jones. Same thing with the samba beat for Sympathy, his musical sensibility was imprinted on all the rest of the group, that's where all those ideas came from. Probably the single most defining aesthetic of the Stones has been their instrumentation, they have texture none of the other hard rock and blues bands ever did. If you love the 4 album run through Exile, then you love Brian Jones.
Even the Stones themselves will tell you this.
"He formed the band. He chose the members. He named the band. He chose the music we played. He got us gigs. ... he was very influential, very important, and then slowly lost it and blew it all away." - Bill Wyman
He was one of those people who are so beautiful in one way, and such an asshole in another.’ Keith had once said to Brian: ‘You’ll never make thirty, man.’ Sadly, Brian’s reply was simple: 'I know.’
A lot of Stones records were built of riffs, and Brian invariably played those riffs.- Glyn Johns
‘If Keith and Mick were the mind and body of the Stones, Brian was clearly the soul.’ - Rolling Stone