Well yes, plastics are a problem - but do you have a link that argues that a substantial proportion of those microplastics are from disposable bags? I'm open to argument on the topic, I'm OK with the plastic-bag levy, but my understanding is there are far more significant sources of that plastic contamination - disposable shopping bags are pretty small proportion of plastic waste. And paper bags have their own environmental costs, which is why the obvious approach is more reuse of bags.
What I'm arguing against is the notion that it's a solution to just exchange plastic disposable bags for paper disposable bags. Paper bags take far more energy to produce, for one thing, which will create further pollution along the way.