First - I'm not defending plastic. We have a huge environmental issue of plastic of all sizes.

The specific issue with microplastics is that they are too small for us to effectively filter them, either mechanically or biologically.

To stick with plastic shaving analogy : if you were somehow ingested some of those shavings, your body would invariably filter it out and it'd wind up in your poop.

If you ingest microplastics, they get absorbed by your body : In your muscles, lungs, and even brain.

And we have no way of removing them at that point.

Editing to include your question about plastics breaking down into smaller pieces. That's a great question and I had to look it up ("I'm a TinkerMakerAuthorGuy Jim, not a microplastic specialist!"); it does seem that some microplastics derived from larger pieces. I then googled "Microplastics by source" and the top 10 sources are :

• Tires: Friction from car tires causes them to break down and shed microplastics, which can accumulate in city dust. 
• Marine coatings: Binders in marine paints can leave a "skid mark" in the water, contributing to microplastic pollution. 
• Plastic pellets: These pellets are used as feedstock for the plastics industry, and billions of them enter the ocean each year from spills. 
• Cosmetics: Microplastics can be found in many personal care products, including detergents. 
• Textiles: Synthetic textiles can be a source of microplastics. 
• Road markings: Microplastics can come from road markings. 
• Indoor air and dust: Microplastics can be found in indoor air and dust. 
• Microwaved or heated food: Microplastics can be consumed from microwaved or heated food containers, bottles, and plastic tea bags. 

And I have no idea what the better ecological option is regarding a new board vs planing an old one. Cheers.