there's very little good jazz that is actually jazz.
good jazz that is actually jazz - miles davis "relaxin" and pretty much anything ellington did - and that's about all. ellington is 100% jazz and is 100% genius.
everything else that is any good is merely music which is associated with jazz: free jazz, electric jazz, soul jazz, jazz-funk, space-jazz. most of the time if it's not essentially soul or funk it's actually rock music in disguise. or in the case of mingus modern classical in disguise.
re: good jazz, i think it was probably equally good to someone in the 40s as to someone in the 70s, but it's reasonable to expect the latter to resonate more with us as we identify with change and take pleasure in progression.
re: what is jazz, if you liken jazz to a chair for example one could make many different chairs and they would all be chairs, even the odd ones. if there is a jazz idea or essence the little sticks, backrest and horizontal sitting place that define jazz as jazz will all be there however weird the form (i think). the advantage of this is that one can overlap with other musical forms and still be jazz, and not fear contamination, which is i think more fruitful than building a moat around a certain form, however perfect.
having said that i do agree that there are moments that define ideal forms along the way, that one could isolate to represent certain variations and perhaps the pure or blueprint idea as in the examples you mention is lost in later variations, but one could equally say that later forms are full and defined and essential, and perhaps a better expression of the ideal form than the early budding stages. perhaps there is more than one perfect form.
one thing that i miss (not only in jazz) is a discussion of or attention to context. with 60s/70s music especially one could argue the evolutions/ramifications of the music are a reflection of general social change rather than something intrinsic to the music, it's a potential realised almost by accident via cheap electronic instruments, eastern philosophy, travel, politics and general throwing the old order out the window. the end result is thrilling and wonderful as in the case of free jazz (brown/cherry/etc.) but also depressing in that this change/liberation brings about the end (or natural conclusion) of the form, the famous 'death of jazz' which leaves us to scavenge the remains or turn into early jazz purists..
perhaps with time these distinctions will fade away into the broader idea of convulsion/liberation from older forms? perhaps improvisation or immediate composition will be the name of all this and the venues and brass instruments and genre minutae of jazz and rock and so on will long be forgotten.