Low Top Daily Ratings

The daily ratings in the leaderboard seem very low; the top is only 2600s. Even adjusting for the fact that the daily pool at the very top is probably worse (there are probably very few GMs there for example), it still seems too low; a 500 elo difference still seems like way too much even accounting for this. I think chess.com tried to have a rating adjustment (such as 2200+s getting 400 extra), but there were problems in implementation and it was undone. The average daily rating does seem reasonable though.

Also, looking back to 2009, there seems to have been tons of people with daily rating 2700+, some up in the 2900s. What happened to all of these players? 

I noticed that. I've played daily against a few players who were rated over 2200 about 10 years ago but now 1600 or 1500 rated, I think its because there were less people on the site back then.