You are right. There are two separate issues involved here. The question of how Jan would be perceived today if he had lived hard and died young is, as you say, an interesting one. But in the end an unanswerable one for the reasons you suggest. Some people become mythic figures when they die and others don't. It depends to a large extent on how your career is perceived at the time you die.
Look at the difference in reaction to Roy Orbison's death and the suicide a year or so later of his contemporary Del Shannon. Orbison was ascending in an unexpected second act at the time of his death (partly or maybe largely due to the Travelling Wiburys) and Del Shannon wasn't. Orbison was feted and compilations were put out after a tasteful interval. Shannon was more or less ignored.
As for Dean, California was *not* unfunded and unsupported. If a group allegedly involving Brian Wilson, Bruce & Terry and Dean Torrence is not capable of attracting backing and support then I doubt that this was to do with the fact that Dean's former (at that time) partner was still alive. By the time he and Jan were back together (for the reasons Mark pointed out above - neither of them could draw crowds or money on their own) Dean had had a window of opportunity longer than many pop stars' careers.
Who can say why this window of opportunity didn't open? All I am saying is that I doubt if it was Jan or the public's fading (at that time) memories of him that kept it closed.
Cheers
Owen
