The Stones, Who, etc. were all talented enough throughout their lineup to go on minus an original member while still being the same band. The Jam without Weller is totally different.
He was the center of the band's creativity and talent, as well as their personality. Foxton and Buckler just kind of followed his lead...Weller went from being a Tory to a left-wing socialist (hi, JDV!)...from being a shaggy-haired rocker to a spiffy mod...musically he went from a Kinks-meet-punk sound to psychedelia to African-American-influenced styles...the other guys just followed along.