My single biggest issue is weird complexity requirements... let me simply use a relatively short sentence (15+ characters). If they limited requirement to length only + a breach check, that would be enough and encourage a sentence.
"I really like sour grapes." is easy enough to remember and has plenty of complexity... of course, it gets much harder on a mobile device, this is where passphrase managers come into play though.
Random words, yes. With random words, you've turned your set of 40 or so symbols into a set of 150K or so symbols, so it's fine that you've got fewer of them in your password.
I'm skeptical about actual sentences. With an actual sentence more common words will be chosen (so a smaller set of symbols), it will have a structure (no need for an attacker to try, for instance 'noun noun noun noun noun'), and people will probably choose from more common patterns: "I really hate arbitrary requirements."
I was just giving an example... there are a nearly infinite number of sentences out there, even if they share a common structure. That said, it doesn't have to make sense and I'd be okay with "noun noun noun noun noun" for that matter. The point is, you hit a point where it's good enough and even then if you really need more security that's where MFA strategies come into place.
"I really like sour grapes." is easy enough to remember and has plenty of complexity... of course, it gets much harder on a mobile device, this is where passphrase managers come into play though.