In the past 20 years h/u's have become all-in-one source units and pre-amp centers. Many decent decks today have more capability than a car 20 years ago filled with external EQ's, xover, power conditions, LOC's, etc. Better DAC setups have also improved sonic performance on the average deck.
In the past 20 years speakers have seen some significant design advancements. Thanks to cheaper amplifiers in recent years, the speaker industry (subwoofers) has been able to adapt designs that excel in smaller enclosures, a paradigm shift that fit well with the tight confines of a vehicle. Subs have also seen a significant increase in excursion capabilities while also adapting a few different new motor topologies that allow the use of that long throw while still remaining linear, producing virtually inaudible distortion levels. BL optimization is a relatively large step in technology in a system that had remained relatively unchanged for decades prior. Several major improvements in speaker design that are beneficial both for sonic performance, and for logistics.
In the past 20 years amplifier advancement has centered around affordability. Companies expanding production to other countries, and the advancement of automatic assembly of computer boards, has significantly reduced amplifier costs. Logistical improvements have mostly been in the form of built-in sound processors, something that is relatively useless in many systems, and flat-out unwanted by some people. Amplifier topology has evolved into utilizing class D architecture, but this has been for logistical reasons (cost, current consumption, heat generation).
Its easy to look past the benefits of modern amplifiers and fall back on personal reasons to go 'old school'. But its not so easy to use 20 year old speakers or h/u without significantly missing recent technological advancements.