Eh. There's absolutely zero reason why they couldn't have had sympathetic underclass characters in the story even if they were morally grey or flawed characters somehow.

Look at the orphanage scene and the narrative it creates around disabled beggar children, for goodness sake.

I don't think that the story in the film would have been somehow diminished if there were other slum-dwellers who were virtuous. Shit, the protagonist could have even paid back those other virtuous characters by helping them out after his win and it would have contributed to the moral justification for his character being in a rags to riches situation. But no.

if one looks at the characters as representing the Indian underclasses as a whole.

I mean, wasn't the movie representing basically every Indian underclass and every trope about the Indian underclasses throughout the story though?

If we transposed this argument to a film that represented queer people or people living in the projects in the same sort of way, for example, I'm not convinced that you'd be arguing against the fact that it's acceptable to have essentially no virtuous characters who were queer or from the projects.