Old lead pipes had hard untreated water flowing through them. The lead pipes would internally (normally) be coated with salts, and the lead did not (normally) leach into water. But soft water does not have calcium or magnesium in high enough quantities. Also, even with hard water, pressure changes could loosen the scale deposits.

Microplastic risk is not anywhere close to lead, we should not even be discussing these two things in the same paragraph.

Lead is bad because it mimics calcium and iron in our body, binding to proteins, sneaking into bones, causes anemia, disrupts brain function...

Plastic is inert, it is made of long chains of carbon, hydrogen and oxygen. These long chains do not break down easily. Microplastic, while it does not pass through the body, and can accumulate in organs, its impact is still under study. We aren't ingesting high doses.

BUT, bad pipes may leach other stuff. Some additives in certain plastics seem to mimic hormones and potentially disrupt them. Some additives are carcinogenic. (but only in high doses I guess). Certified modern pipes are safer.

Yes, depending on alkalinity and so forth --- see Flint, Michigan in the news a while back, or on-going efforts to remove solder containing lead from the plumbing aisle.

Yes, the only reason we are ok with some old lead pipes left is because they're coated in scale build up. As long as that's not disturbed it's not dangerous anymore.