Why is my plumber suggesting that I replace the poly water supply line to my house?
I have a feeling that we're actually talking about polybutylene, which is obsolete and could account for the confusion. I think most of us assumed you're talking about polyethylene, which is commonly used today.
Your home was built during the window in which polybutylene pipe was used for runs from the street to the meter. It was discontinued due to lack of durability:

While scientific evidence is scarce, it is believed that oxidants in the public water supplies, such as chlorine, react with the polybutylene piping and acetal fittings causing them to scale and flake and become brittle.

This would also explain your plumber's suggestion, which seems reasonable in that case. Due to slightly different wall thickness, commonly available fittings cannot be used. Any plumber who cares to stand behind his/her work may also be reluctant to work on PB, since it's more likely to result in a callback. You might point out that SharkBite offers transition fittings, which can be identified by their gray plastic sleeves. So a solution may be at hand.
&quot;Black poly&quot; isn't really a thing, officially. It's a layperson's term for an unknown plastic or a tradesperson's nickname for polyethylene that happens to be black. Color is irrelevant with these materials, though.