I think you vastly underestimate how much plastic we use and how optimized it is.

The global production of new plastics is a few hundred million tons per year, not including textiles (elastan, nylon and anything polyester is plastic). Then we have a ton of different kinds of plastics, mixed with thousands of different other components, to get the right results. A producer of packaging machines once said their most complex machine is for wrapping cheese, as that were like 5 layers of different plastics with different functions. Masks and other PPE is also mostly just plastic, and one of the really resilient kind.

We can substitute a lot of plastic, or simply not use anything as a substitute in some cases (thinking of the image of peeled oranges in plastic containers here, thats a complete waste), but that is way harder to do than people like to think. There is another reason than just cost why we use plastics.

You don't usually use microplastic, you use plastic and that just breaks down over time. Just the abrasion in a washing machine is enough for that, but also basically any other mechanism that degrades the material.

Microplastic to plastic is similar to sand in comparison to solid rock.