Top gear was barely around before Clarkson, he started working on the show in 1988, well before it was a popular show, and he has always been a host in the current top gear (considered a reboot starting in 2000).JoJo said:Top Gear was around before Jeremy Clarkson, given the show's popularity I don't doubt the BBC would at-least try to keep it around even if all three current presenters left. Whether it would survive for much longer or quickly lose it's audience is anyone's guess, depends on who they would hire as a replacement really.
If BBC do fire him, they will likely attempt to cast someone else, but I do see Richard and James both leaving along with Clarkson and it would take them all but 3 seconds to be receive hundreds of big money offers to do a top gear-like show.
Even if Top gear dies with Clarkson, there will no doubt be a Top gear 2.0 featuring Clarkson just around the corner.