I see no problem at all with trying out 2003 technology in your old JBL loudspeakers. Just a few responses to some of your statements and answers to some of your questions though -
"JBL cut corners on hardware and internal componets. I, C, and R in the XO's were not up to what these speakers could and did deliver."
They were fine for that era.
"IMO, R is sonically destructive in a crossover. It should be used as little as possible, unless you like a veil between you and the music."
Capacitors and inductors are far more troublesome than resistors. They, and not resistors, are the truly pathetic passive components.
"it became clear the XO slopes were more for withstanding abuse and protecting the horns from distorting"
The 4343's are Studio Monitors. The higher order filters do offer increased protection and bandwidth limiting but they also offer greater control over dispersion characteristics and that is one of the design criteria of the Studio Monitors. They didn't want vertical dispersion running completely amok. More recent JBL Studio Monitors employ 24 dB/octave filters in order to tightly control each transducer within its intended bandwidth.
"Why would anyone use electrolytic caps in a high performance XO?"
Because they were a viable solution 25 years ago. Just like using a 2231 or 2121 was viable. The newer 4344 and 4345 used newer technology and paralleled mylar capacitors bypassed with polypropylene capacitors to get the necessarily large values. They also used the newer 2235 and 2122. The electrolytic capacitors, the 2231 and the 2121 were no longer viable solutions for the task at hand.
"And after 25-30 years, do you think the 120uF value is still stable? No way."
And neither are the compression diaphragms, nor the spiders and compliances, nor the magnetic assemblies. Stuff wears out.
"All I can say is the OEM XO's are slow, veiled, garbled, soft JUNK."
They were excellent for their era.
"What a travisty to put this in front of such wonderful Alnico compression drivers."
It isn't a travesty, the 4343 was nothing short of excellent for it's era.