I have vinyls of both periods, and i'm not sure on what you can name as "thin".
I bet it's a total personal mater of taste, and you prolly gonna make fun of me, but I find that Prince's Lm-1 is sounding more punchier and warmer than John Bohnam drum kit
. I was raised on New Order Blue Monday beat, and maybe I spent (and still spending) far too much time in nightclubs, but I usually find 70's record very poor on the kick drum. Well sure I more than love the thing labelled as Synth Pop, and it hardly existed in the 70's, exept maybe Giorgio Moroder or Kraftwerk. I do own all Roxy Music LP's as well as Bowie's till Let's Dance, some Led Zep and the fantastic Chic ones. The Chic sound is one of my favorite ever, especially the Bernard Edwards bass sound, and way of playing. As for Roxy Music, the early work sounds amazing (Bogus man), but to my ears less warm than Avalon.
As for a typical 70's hit, I can think of Abba's Dancing Queen, and I find the bass and drum parts very "thin", the overall mix sounds too much bright to me, but it's multi platinum isn't it ?
You gonna think I'm a freak but of all songs and record I own and have listened to, I haven't found anything as warm to my ears as Billy's Idol Eyes Without a Face, just love that bass sound, whenever I play it it does dudududududum to my belly, I think it was mixed on a Neve (and yes it's a LinnDrum).
Then I do agree some things sounded really "poor" and thin, though we may not have same references as North America and Europe were different markets. But I own a bunch of italo dance records which were summer hits in europe and are obviously low rent gear, studio and personnel. So maybe that's where it is, as 80's went by, a lot of affordable japanese recording gear and instruments were on the market for some wealthy hobbiests with some primitive knowledge in recording/engenering/mixing and voilà...but that doesn't explain your point of view on Appetite for Destruction which I too own on vinyl and wasn't low budget. It doesn't sound "thin" to me, it just sounds "metal" though i'm not a metal expert.