I have researched the topic of sustain in so much advanced ways (physics, vibrations), that I think that someone trying to explain sustain problems by blaming the fretwrok "silver-bullet"-style, and bringing the fretwork in any discussion about sustain, is just downright retarded.
I said it could be the problem and I'd look at it. I don't know whether it is without seeing the guitar, but it is the #1 sustain issue I see.
I work on this stuff ALL THE TIME and my specialty is getting guitars to play clean with action most people say is impossible. I know WTF I'm talking about. You, on the other hand, have no idea.
Fretwork does not have to "buzz badly" to be the culprit. Not at all.
There is so much wrong in your posts it would take too long and be a fruitless exercise to correct. Try actually doing some fretwork for a decade or 2 then post again.
That is the problem with forums. People who don't actually do this stuff day in day out want to pop off and talk about things they know nothing about, and will argue with you til they are blue in the face with incorrect information. Its like a layman going on a surgery forum and arguing with surgeons about brain surgery. I try hard not to get into it with the ignorant because I know how useless it is to try to explain relativity to a chimp. The chimp just gets upset and the explainer just gets frustrated.
The amount of BS by "professionals" on the net is unbelievable. All the fretwork of the world COULD NOT SOLVE SUSTAIN PROBLEMS ON THE LAST FRET FOR JESUS CHRIST'S SAKE! Doesn't take a rocket scientist to see that.
I now have my Ibanez's action on 0.9mm high E @24th fret, 1.3mm low B @24th fret, with 4 seconds of sustain on 24th and more than 5 secs on 17->22.
Thing is that I SOLVED MY PROBLEM, by experimenting with the amount of body wood under the heel, offsets, neck heel position, screws. If I trusted "high-school"-level "techs" (like you) they would drag me for years without any solution.
Truth is my friend, that it is apparent that your knowledge, and imagination are both limited. You parallelize yourself with doctors, which is again laughable. (You also mention relativity in this context which made things more funny, since all you do is repeating the word "fretwork" like a holly grail of sustain, not very scientific ). I gave the link to a scientific paper on sustain before, but since your lazy finding it here it comes again, (happy reading) :
https://www.unibw.de/lrt4/mechanik/mitarbeiter/ehem-mitarbeiter/hfleischer/deadspots-en( I am sorry to disappoint you but you won't find the keyword "buzz" or "fretwork" in the above doc).
After you understand this, come back to talk about sustain, after you promise that you won't bring up this idiotic argument about fretwork again.
In short :
any vibration of the medium (neck or body/bridge) at a frequency near (or a multiple of) the fundamental string frequency at a fret which constitutes an anti-node of the wave, will most likely result in very poor sustain.
The way to solve sustain problems is to minimize those vibrations, while not sacrificing (e.g. block the trem) the functionality of the guitar.