I didn't live back then (80s), but I think it comes more down to the idea and film they are trying to make. Do you need nudity? Some films, like AMERICAN PIE for example, would seem odd without it. Other films it's just not called for. The 80s seemed, at least to me, more about pushing the envelope. Now that that envelope's been pushed, there doesn't seem to be as much of a call for it. Action on the other hand - it seems we've gotten to a point in movies where a lot of the action on screen is more fantasy based so that would naturally decrease the rating. Kill a monster graphically? All is good. Kill a human graphically? Better have a damned good reason. Would using the R word more than once improve the film? If so, use it. If not, what's the point?
Basically saying I can see how some people are upset that there are less R rated movies these days. But you also need to ask yourself what, if anything, is lost? Movies come down to the execution of story and character, not nudity or tons of gore.
Those angry about PG-13 movies to me it seems to come down to two camps - those who want to see nudity and those who want to see more gore, rather than caring about the story itself. A horror film can be just as scary as a PG film - so the problem there isn't even the rating, it's the writing and the execution of it. If the story calls for R? Then, yeah, it should be. If not? You're just selling out. Yes, seeing the studio system from the inside sometimes nudity is just added in with no concern to the story in hopes that it will draw more people in, same with everything else.
I think the thing here is film makers, and studio heads, want to reach as wide an audience as possible. If you can make a film PG-13 and lose absolutely nothing besides some quick cuts of gore or covered up nudity, why make it R? Plus, over the years, I believe due to pushing the envelope more and more - films that once were rated R, if released today, would probably be PG-13 without alterations. The MPAA is more easily manipulated, it seems, than they once were. Look beyond the rating itself and you'll probably see it's more lax or just less gratuitous since that envelope's already been pushed and nothing would draw people in from pushing it like it used to.