Well I don't know of people claiming that microplastics are "killing us", there are dozens of papers that implicate microplastics in negative health effects from hearts to intestines, to sperm.

https://www.nejm.org/doi/full/10.1056/NEJMoa2309822

https://pubs.acs.org/doi/10.1021/acs.est.3c09524

https://pubs.acs.org/doi/10.1021/acs.est.1c03924

https://www.thelancet.com/journals/ebiom/article/PIIS2352-39...

There are a lot of studies that find correlations, and then are studies like this one that show that the direct introduction of microplastics alters cell functions negatively:

https://pmc.ncbi.nlm.nih.gov/articles/PMC12692081/

I think at this point we should stop talking about how "there's no data" or "no studies" and "no one has shown" and graduate to "oh, maybe should figure out the extent of the damage."

Microplastic pollution is a global problem amongst a whole host of global pollution problems. We'd do well to try to figure out how bad it is, because it isn't going away. Oh, and we should probably work on fixing all of our pollution problems, especially cumulative ones like this.