It's a vicious circle, these past twenty years. Before then, movie makers made effort to ensure dialog could be heard and understood. There's a line in Casino Royal that James mumbles, "He's a bloody idiot, then", but you can still understand him, it's meant to be mumbled, Eva Greene asks "What?", but he's understandable when he's acting like he's mumbling. They wouldn't even make him unintelligible for a line that was meant to be unintelligible.

Without subtitles, when I don't have them for whatever reason, sometimes I need to rewind, and turn up the volume to catch a single word the actor mumbled. Sometimes, I never hear what they said, and I just move on. Was that important? I don't know.

We started watching with subtitles because we couldn't hear what the actors are saying.

"Big explosions go boom, everyone likes loud music."

Actors don't need to speak clearly, which adds "atmosphere" or whatever, because everyone is watching with subtitles.