If Delta wasn't selling enough D1 seats to make them profitable, I'm sure they'd remove seats -- in fact, didn't they do that a few years ago (maybe when EC came around?)
I don't do D1 as much as many here as I travel completely on my own dime. But I have used all my GUCs, gotten a share of OpUps and bought the occasional discounted Z fare.
I can remember only two times where other passengers behavior rose to the level of disturbance. One was on a 767 where I try for 1A or 1D for the larger footwell. There was a party going on the fwd galley -- I'm assuming the passengers were DL employees since they were talking with the FA. The champagne was flowing (not by FA's) and it got so loud it woke me up. I went to restroom and just said off hand "Kind of hard to sleep" and things quieted down.
The other case it was a group of younger (20-something I assume since alcohol was being consumed) who were basically having a party in the cabin. I have no idea of they were non-rev or not but I got that impression from something they said (can't remember what). The FA finally came back and read them the riot act and things quieted down.
I've had noisy people when flying in the back as well. That is life...