I think the vast majority of improvements have been in materials development. Some very high tech and some not so much. I worked for AR back in the late 80's and we did a HUGE business replacing drive units who's foam surrounds deteriorated. This is mostly due to UV, heat, and humidity. And it wasn't just production speakers. We supplied Mark Levinson's Cello Amati with the same bass driver from the AR 9. We literally were sending container loads directly from the driver manufacturer in Japan (Culver Tonnegen) to our Asian and European distributors. In addition, the electrolytic caps were long term time bombs and film caps were only put in high-end models back then. We were just getting into light metal domes back then (Ti mostly) and quite frankly, they sounded wretched to my ears. Today the designers have so many more affordable options.
Sound quality of Newer versus Older speakers
From a sound quality perspective, is there anything that newer speakers are doing better than older speakers. For reference, I have a pair of Monitor Audio Silver 300s which are amazing me with their ability to balance detail retrieval with an ability to avoid harshness (with the right ancillaries). My subjective perception is that this type of balance between resolution and refinement was more difficult to find in speakers from 20-30 years ago.