Yes, pollution is bad, but in some ways it's good that the plastic is getting spread out. Low levels spread across a wide area makes it more likely some bacteria would evolve to be able to eat it efficiently.

Of course it still sucks if you want to drink the water or breathe the air and there are all these polymers in them.

It would be quite a bit better if it was all in one place so the bacteria we are engineering to eat them have one particular place they can be deployed instead of the plastic microparticles and polymers wrecking havoc everywhere. If all the plastic was just stable in a giant landfill, it wouldn't even be that much of a problem/a significant ecological risk.

It isn't clear to me quite how risky these plastics are - is a grain of plastic worse than say a grain of sand?

Sure, the sand has more density so usually settles sooner, but apart from that both are effectively biologically inert.

Plastic is not biologically inert. Micro plastics accumulate inside tissues and many plastics contain endocrine disruptors, including some non BPA plastics. Glass waste (basically just sand) is not a big problem pollution wise since like sand it's basically just a rock, but plastic does break down slowly, making it far more of a concern.

Sand and rocks break down slowly too, and frequently give off heavy metals, arsenic, etc. in the process.

What's key is defining the magnitude of the harm of each, and just stating 'X million microparticles' doesn't do it.

Some stay dry and others feel the pain...

Microplastics running through our veins