Danny's stats were mediocre in large part because he was the fifth option (through no fault of his own). Later in his career (particularly in Sacramento) he was more of an all-around player, who played at an all-star level.
As for the argument of "what if the Celtics collapse from here?", I don't really care. The three year run we've had (regardless of what happens this year) has made Danny an overwhelming success as a GM. He came in and transformed a mediocre team in decline into a title team, through great drafting and smart trades. If the Celts never sniff playoff success under Danny's guidance again, he's still done a good job, and a better job than most.
A clearer argument for why it is completely premature to discuss raising him up with Auerbach, Bird, et. al. could not be made:
5th option (or higher) as a player; good job, better than most as a GM doesn't exactly cut it.
When you say 5th option, though, you need to add in the qualifier "on the team that was widely regarded to have the best starting 5 in the history in the league".
BOOM! That's what i'm talking about. TP BBallTim