Slash was popular, but Axl Rose was the figure head of the band. The lead singer is usually the most popular. It is true that Slash's guitar playing is legendary and that he lends a very unique and distinct sound to the bands music, but it can and did go on without him.
Axl Rose is the front man for Guns N' Roses and is the most integral face to the group. While Slash might be very popular, Axl was the looks and the voice to the band. Even after many years in the limelight, people who aren't even that familiar with Guns N' Roses remember Axl's face.
Yes, I feel like Axl Rose was a more major player in the success of Guns n Roses than Slash ever was. Axl Rose is a great musician, and is extremely popular. Slash is popular also, but he did not stay with the band as long as Axl has been.
Guns n' Roses are still in existence today, and has been going strong for decades with him and the front man. Slash was definitely a huge presence on the stage for the band, especially with his easily identified image, but he's been gone for years and the band has gone on.
Slash was just the bass player. Axl Rose was the lead vocalist and front man. As such, Rose is more integral to the success of the band due to his distinctive voice. A band is only as good as its voice, and since there was only one lead vocalist for Guns N' Roses, that voice carried the band.
And bands work together when they compose songs. It was often that Slash and Izzy were just jamming when Axl wrote lyrics to what Slash and Izzy were playing, like Sweet Child O' Mine. Or Axl came with a full, self composed song, like November Rain. So they worked together.