Galvanized pipe life expectancy

USA, NW Oregon, single-story house with crawl space, built 1972.

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Background

The house has the original galvanized piping in the crawl space and suffers from low water flow in two bathrooms. Static pressure measured at an external hose faucet near where the service line enters is 80psi so pressure is not a problem. The kitchen, which is next to the service line entry, does not have a flow problem

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The home inspector suggested the galvanized pipe is near the end of its service life. An additional factor is that that the previous owners did a major remodel of the kitchen and both bathrooms 5-7 years ago and installed copper risers (yay, no sheetrock tearout to repipe) but the contractor didn't use dielectric couplings, which could have accelerated corrosion in the galvanized pipe near the joints. When I replaced the shower heads I found some black crumbly granules in the old shower heads, and also in a sink aerator.

We have decided to replace the galvanized pipe with PEX, and there is some disagreement among contractor bids about the necessity of replacing the galvanized service line from the meter to the house. One contractor thinks it is not necessary.

Questions (all related):

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1. Is the expected lifetime of galvanized pipe in the 40-50 year range as claimed by the inspector?
2. Could the lack of dielectric couplings have greatly accelerated the corrosion leading to the current situation?
3. If yes, would the copper be corroded/weakened as well?
4. Is there a way to inspect the service line to determine if it needs to be replaced?