So, let's break this down...
If Danny didn't trade Perkins, they might have saved last season (who really knows, and its not to relevant to this discussion), but it would have made little difference for this season, since he would have just signed with OKC or another team willing to give him a huge deal. Perhaps Danny could have been able to work out a sign and trade for a TPE, but that would be the best they could hope for, and that wouldn't have been a given.
Losing West was not Danny's decision. West went to Indy, because their offer was going to come off the table if he waited for the C's, and the chances of the sign and trade getting past NOH ownership were pretty slim IMO.
Semih has become one of the most overrated players in Celtics history. I think Stiemsma is better than him, because he can actually defend the pick and roll.
Allen would be nice, although I think he would have been just as infuriating to watch (and coach) as he was for all his years in Boston...he just did not fit into their system.