Good point. I don't think those issues cannot be resolved though, but it is one of those things that might not be as cut and dried as it seems. I wonder if biodegradable or recyclable bags made of paper or something else are the best of both worlds?
There are likely to be longer term positive effects of banning plastic bags as well though. Plastic bags don't biodegrade, but they do seem to break up into little bits and can make it into the food supply through fish and maybe other animals. I have a hard time believing that plastic bags as they are are an actual good solution to the problem of needing to get food to your house.
To me, the ideal solution to plastic bags is the same as the solution to most environmental problems: correctly account for the cost of the environmental damage and add that to the cost of the product (the company can chose to eat it or pass it on to the customer). I know that correctly calculating the cost is probably tricky but still. That would fix a lot of our environmental issues. Gas and coal are only cheap when we don't factor air quality into the equation.