Extremely anecdotal, but if I compare top 40 pop from the early 2000’s (Black Eyed Peas, Anastacia, Maroon 5) with stuff from today (Ariana Grande etc.) then the old music sounds like boomy maximalism while the new music usually has crisper drums, more subtle details etc.
A lot of present-day mastering can technically remain loud while sounding a lot less harsh. That's thanks to aesthetic mixing choices and at least in part due to audio processing and tools like plugins from Oeksound (Soothe) and others, which are used on nearly all top-selling music now https://oeksound.com
I find that the likes of Ariana Grande have a lot more air in their music those 00s acts you describe, while still being quite loud. So do my absolute favorite artists Tame Impala, Caroline Polachek and Dua Lipa.
Commercially this probably works out with better headphones and earbuds as a default listening mode, as opposed to things like the 00s earbuds delivered with digital music players and early smartphones.
That first Billy Eilish record would probably not have been a hit if the target audience was getting their first impression of new music in cars driving at highway speeds.
