This goes back to what I said about accounting tricks and environmentalism–if your focus is on carbon footprint, you'll get one answer; if it's on other pollutants, it's another, if it's sustainability, it's another, and if it's waste, it's another. A flimsy reusable bag may have a negative impact on climate change, but if my focus is on reducing the amount of waste that goes to landfill, it's a better option. Even if current studies may say microplastics are not a cause for concern, someone may still object to creating them, either because it's possible that the science is not yet advanced enough to pick up the full effects (i.e. it may cause a problem later on down the road that we just can't see coming yet), or simply out of a moral belief that unnecessarily introducing synthetic materials to the native environment is a bad thing.
And it's also about the particulars that go into making an individual object. Making a paper bag, for instance, takes up a decent amount of energy, but if that energy is produced by solar or wind, then it doesn't really matter. Some wood pulp providers clear-cut virgin forests, others only take x% of the trees and replant new ones to replace what they cut.
As regards reusable bags, I would think 131 uses is easily within the realm of possibility for a good-quality cotton or canvas bag. Assuming you only go shopping once a week, that's roughly two and half years of use, which doesn't seem impossible. And I occasionally use my shopping bags for toting things around the house, which isn't something I typically do with disposable bags.