This.
Popular music existed in their days, yet they chose to write something different and, actually, much more complex. There’s absolutely no reason to think they’d be enthusiatic about rock or modern pop. Harmonically and melodically, these genres would have very little appeal to them: they’d just be way too simplistic. That doesn’t mean they wouldn’t use a modern pop melody (they did occasionally use popular melodies of their times) but I bet it would only be as a basis for much more ambitious and complex works.
Now, they’d probably be fascinated by advances in technology (recording, electric instruments) and they’d probably incorporate them in their work. Composers are often interested in new sounds or combinations of sounds (reportedly, Brahms was delighted to hear a banjo towards the very end of his life), so they’d certainly give them a try.
However, I’m really uneasy with the concept of “musical progress”. What does that mean exactly ? That today’s music is better / more complex / more advanced than what came before ? Put that way, I think it’s grossly wrong.
In terms of technology, certainly. But technology =/= artistic quality. You can have amazing tools and still do a crap job.
In terms of rythym, it’s debatable. Certainly, today’s music is a lot more heavily syncopated than classical music. Is that more advanced ? On the other end, you could argue that having drums everywhere is actually a regression: are 21th century people so musically-impaired that they need a bang on every single beat to feel the pulse of a musical piece ?
Harmonically, well the less controversial thing I’ll say is that it’s different. Put Pérotin, Beethoven, Ferneyhough and, say, Dylan together and I don’t think they could meaningfully collaborate, at least before a very long period of observation and tons of explanations. These musicians follow different rules (yes, even the most revolutionary rock / pop musicians follow rules and patterns, even if unconsciously) and, perhaps most importantly, have completely different goals.
As for “fairly rigid rules and musical structures”, well I’m not sure that classical music is the most rigid of these genres. Take just about any work by Bach and see, for instances, how it often modulates, venturing not infrequently into some pretty remote territories. An awful lot of rock and pop songs stay boringly in the same key from start to finish. And when you find a modulation, it’s usually a rather “tame” and expected one.
Or take musical forms. Yes, symphonies, sonatas and concertos have a given structure. But, lots of composers started experimenting with those structures pretty much right away, litterally exploding them within years of their invention - Beethoven being an obvious example. Moreover, the overwhelming majority of rock and pop song are also very rigidly structured: Verse 1 - Chorus - Verse 2 - Chorus - Bridge - Guitar solo / rap bit - Chorus. Of course, some rock and pop musicians have broken this canvas but I think people vastly underestimate the number of classical musicians who did the exact same thing, centuries before. They weren’t as constrained as implied here and, conversly, modern pop musicians aren’t as free and as revolutionary as they believe.