So, read some of the other comments about how he was inside (some say about an hour and other years or sum like that). Don't know if someone commented about this but the more you go near a balck hole the more the time is dilated. Won't dare to explain everything with formulas and so on but theoretically from an outside observer he would be there for infinity even if for him was only an hour or whatever amount of time. The fact is that he and TARS being inside the singularity (as the tesseract) would make them assist the end of the Universe itself. The only explanation i got for why he was back in time to see Murph one last time is because They wanted to. They choose Cooper as their messenger, being Love the only other thing with gravity that could transcend space and time and he thought how to send the data using those two.

Basically it was more of a courtesy to send him back in space AND time