Blimey, what a complex and subjective subject: cor love a duck, guv!
I would love to know how many of the 30s - 50s actors spoke in their original voices. Cary Grant, Bob Hope and a few others were born British so tbey would have reached Hollywoodese from the other side of the Pond from those born in America (what percentage of the "stars" of that period were actually born American nationals I wonder?)
Maybe there was a time when, after the Marx Bros and similar, the market for American films started to open up, and maybe more British films were flowing the other way. Due to this, perhaps, where strong accents where not actually needed by the story line or location, the blander mid-Pond accent fell better on European ears.
The tone of voice seems to go in fashion, to my ear American "public" voices tend to be lower in tone than European ones. Deeper voices tend to be "seen" as more "serious", more authoritative, than lighter ones - in both gendres. Drawls impart a certain quality to a voice, often sarcastic, whereas its close cousin, the "croak", was once described as imparting a feeling of trust by a voice coach in an interview I once heard.
The basic voice quality might have more influece on the emotions of the audience than how it is used. A very important tool for the actor's. If personal qualities; goody, baddy, snidey, patronising, weak etc, are attached to certain kinds of voices or accents then a bland one may be more flexible and popular.
I have noticed that there used to be a lot of films where the more sarcastic the character the closer his or her accent would be to "educated" English.
[Based on nothing but a fairly casual interest in language and years of "observation."]