Aren't a lot of those microparticles just sand? Asphalt is basically just rocks/sand and some tar-ish cemicals to bind them together.

No you should assume it’s a problem until proven otherwise. We only have one environment and shoving anything we please into it without so much as a second thought is exactly what’s led us to our current predicament.

we know that the microparticles are a problem - the assumption that I'm challenging is that plastic is worse than conventional asphalt.

>until proven otherwise a.k.a. testing

Most tires are made from synthetic rubber which also derived from oil.