This situation definitely would have turned out very badly, due to the high density of population and buildings, etc. in the area, if they had been a little lower or "dirtier" when the damage occurred... although in that case, "Sully" would probably have tried to ditch in the water immediately surrounding the airport (the upper reaches of the East River, where it connects to Long Island Sound).
And if they had continued to try to make two left turns to land on 04 at KLGA, it probably wouldn't have worked.
But it's not like the controller was making rash assumptions- he was only trying to assist the PIC of the flight- who has the final say in any case.
If you examine the transcript closely, you see that the Capt. was the first to come up with the idea of returning to KLGA... so, despite being told they'd lost thrust in both engines, the controller, figuring they knew better then he did what was possible at that moment, gave them a heading to set them up for an emergency approach for Runway 04 (which they had just taken off from).
But less than a minute later, the Capt. informed him that it would probably not work... he was already considering the river (that's a glider pilot for you...
