To a point, I sort of understand the whole, "Get over your nostalgia and get with the times" argument.
It's understandable, but not reasonable. There's no need to get with the times if you're happy with what you already have.
Claiming that a 1970s stereo with scratchy speakers and 8-track player is objectively "better" than a brand new set of high-end Klipsch speakers attached to a top-of-the-line Denon receiver is, on its face, ludicrous. The only reason for making such a claim really would be nostalgia----the memories of listening to music on that 1970s stereo trump the actual equipment used during the experience.
But it's a bit more nebulous than that with an RPG system, for the simple fact that the emotional component is really at the heart of gameplay. Comfort, familiarity with a system and its tropes, experience working through its mechanics, all may lead to the "right" experience where the emotional resonance trumps everything else. An RPG produces an experience, it isn't the experience itself.
That doesn't seem completely absurd on the surface - in fact, I think I used an argument like that to justify running AD&D instead of GURPS (though, that was just to spare the feelings of the GURPS fan in question) - but it's an unnecessary rationalization. Preferring the older stereo you already have is reason enough not to buy/learn the knew one.
In this case, a highly modded game of 1e would be the equivalent of someone taking that 1970s sound system and making their own self-made improvements---"See what I did to the speakers there? And how I soldered this diode on to the circuit here? That improves the sound reproduction."
Yep. And it would be a lot of work to mod a brand new one to get the same results - if it were even possible to mod the brand new one without wrecking it. That adds another dimension, it's not just nostalgia for the time and the experiences of using it, it's the personal investment in modding it.
It's wonderful stuff to be able to go back and enjoy something like that. It doesn't need to be defended or rationalized, even if there are those who don't 'get it.'
what is it about 4e that evokes such passionate responses from proponents?
I can't speak for everyone who ever spoke up in defense of 4e, but, my answer can only be 'nothing.' It wasn't 4e that prompted me to point out that any given h4ter's criticisms were invalid, exaggerated, or flatly false. I'd spoken up in defense of 3.x, of D&D in general (the height of the 90s Role v Roll controversy), and of 5e, now, in response to such criticisms. Rather, there was something about 4e that inspired it's detractors to launch more, more furious, more protracted & persistent false, invalid, exaggerated, and - as you put it 'passionate' - attacks than other editions. The h4ters' very unwillingness to put forth more honest or measured criticism leaves the exact reason for that 'passion' a matter of speculation to this day. When asked, h4ters merely repeat their edition-war-era talking points.
By far the most charitable of possible excuses for their behavior was that they were just being overly reactionary and nostalgic, letting their love for some past edition cloud their judgement concerning the latest one. Were that entirely the case, we could have expected a similar deluge against 5e simply for being new. That hasn't been the case, so the 'it was just nostalgia' hypothesis is questionable.
In any case, nostalgia remains, by itself, without any further justification required, a perfectly valid reason for preferring an older version of something. I unabashedly enjoy 1e AD&D, original Gamma World, and RQII because they evoke the fun I had with them back in the early 80s. That same nostalgia is no small component in my enjoyment when I run 5e, as well, since it very successfully evokes some of the feel of AD&D, for me. I don't have to pretend that any of those games are any 'better' than they are, I don't have to make up imaginary 'play styles' that only those dated systems can handle, I don't need to manufacture faults in newer alternatives - my history with those games is reason enough for me.