JKD is certainly just another variant using common tooling, but the use of beats and measures combined with strong side lead make it somewhat unique. Nothing has come down the pike that is comparable.
Lee not only demonstrated the speed was there, so was accuracy, timing, and an ability to stack up energy behind it. To this day, if you know someone getting into boxing, kickboxing, MMA, 52, etc who struggles with developing power, a copy of the Tao of JeetKunDo will improve this in short order.
There is a trend currently to view Lee as little more than marketing and athleticism, yet to a man none of his contemporaries that spent time with him felt they could best him. This includes guys like Joe Lewis and Chuck Norris. While probably not "the greatest" he is definitely an icon and standard of sorts, the real deal.
Depends on how you look at it, I suppose. The use of beat and the strong side lead both were borrowed from fencing, and 99% of wrestlers lead with the strong side. Mark Hatmaker for example als argues for the strong side lead for MMA, but from what I can gather, he does so also because he takes wrestling into account. I haven't made a study of how it is currently handled in MMA, but I know for example that Randy Couture kept his wrestling stance (he was one of the fairly rare people leading with the left leg in wrestling, but favoring throws in a counter-clockwise direction, which is typical for righties). Then we have a number of more traditional styles where the lead is constantly changed (Silat for example), and occasionally changing the lead is also a thing in old-school boxing (aka "Fitzsimmons shift") as well as modern boxing ("switchhitters") and wrestling (e.g. Abdulrashid Sadulaev). That being said, I am not aware of anyone in top competition becoming suddenly successful by switching leads.
As for what exactly Lee demonstrated, we could argue for a long time. The fact remains that we have mostly demonstrations to work with, and basically no competition footage, so in the end we will never know how well he would have done under pressure. I have seen some clips that look like sparring (1967 Long Beach), but I canot say I have been very impressed by them - then again, some people on youtube seem to consider it an almost divine revelation. I come from a grappling/wrestling background, with some MMA, traditional and Thai boxing on the side, and can't say I found anything I consider to be ground-breaking in ToJKD; if I did meet someone struggling to develop power in a striking discipline, I would tell them to either talk to their coach or find another coach, because I have serious doubts whether reading a book can fix something a coach cannot. And if I had to recommend a book on developing power for boxing, I would probably lean towards Jack Dempsey (who, incidentally, was also among the sources Lee used).
As for his contemporaries, I seem to remember Joe Lewis did say he could best him (but leaving open in what context), and LeBell arguably did (although he did express a high opinion of Lee as a martial artist). And that doesn't include the people who arguably were the top of the crop in combat sports back then, even if we only look at the lighter weight classes, like Roberto Duran in boxing, Dan Gable in wrestling, Tokio Hirano in Judo (although Hirano basically retired to coach already in the late 50ies, so a bit early for Lee), or the people who were doing BJJ and Lucha Livre in Brazil at the time.
So in the end, I must admit I do think of Lee as largely if not completely untested as a fighter, and therefore would prefer to withhold praise of his fighting abilities that might be undue. He was also never tested for PEDs, and due to certain allegations, that could be a factor in comparison. That doesn't change that he was probably still the most influential figure in all of martial arts (but not neccessarily combat sports) in the 20th century. Whether that is deserved or not is another matter, but to be honest I cannot say I really care to go into that discussion.