The difficulty is that it can be both. Dollar stores almost certainly do SOME kind of good, otherwise they wouldn't exist. Unlike SV startups, I doubt the companies behind them are interested in burning piles of money without turning a profit. They ARE turning a profit, which means people are coming it, which means people find the service offered attractive.

That said, people also find the service offered by drug dealers attractive. And yet if the number of crack dealers in your neighborhood explodes overnight, it's unlikely most people will see that as a positive development. A healthy neighborhood is not one riddled with drug pushers, and a healthy economy is not one riddled with dollar stores. There's nothing wrong with a few discount stores, in the same way there's nothing wrong with someone stopping at a fast food place every once in a while. But if you're eating every meal there, something isn't right.