For my money, it's as fine a film as was ever made.
And I find the end consistent with the rest of the play, unlike a lot of the reviewers here.
In an early scene with the girl, Murray's literally up against a brick wall, and he tells her that he can't give in to the welfare people because the boy hasn't yet shown the backbone to stand up to the phonies in the world. He can't let Nick go, because he's not sure Nick's ready to face the world without him.
The scene toward the end - magnificent scene - in which Nick tells Leo Herman what a dip wad he (Leo) is, and how bad his potato chips are, is the boy coming of age, and standing up to the phoniness. At that point, it's fine for Murray to get a job and prevent the welfare people from taking Nick. Murray has accomplished what he wanted to do. Nick's come of age.