I'm sure one of the main reasons they got Rasheed, outside of just plain common sense, was because Darko wasn't an immediate stand out or reliable option at the 4. So obviously if Melo was drafted, there would have been an even greater need for a power forward.
Pistons still would have won that year because worse case scenario LB would just bench Carmelo like he benched Darko, and its not like Ben, Rasheed, Rip or Chauncey would've given a shyt. Tayshaun would probably be happy as hell tho.
But I don't think it would come to that. Carmelo would've been a decent to solid contributor with some good moments, some bad moments and no doubt some ugly moments where he'd get benched for rookie mistakes, missed defensive assignments, being late/hung over, etc. But all in all he'd be no worse than Rondo was for the '08 Celtics. i.e. if he's doing great, he'd stay in but if not coach ain't gon entertain.
The big question would be what would've happened in year 2 as far as Melo's development... but even then, Larry Brown was going to leave at the end of that year.. Flip was going to take over and make them more offensively oriented.. and extending Melo's rookie deal would probably have been seen as the excuse/blame for why they couldn't afford to keep Ben. At some point, the final chapters of his stay in Detroit would prob mirror how it was for denver.. but probably in a slightly more positive light b/c of all the playoff success he would've had with Detroit playing in the east in that time span.