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Yes, with 100% coverage you will write some tests you don't need. Unfortunately, the only reliable way to determine which tests you don't need is to write all of them, then wait 10 years or so to see which ones never failed.

Maintaining a lot of tests is not usually problematic. Many teams have automated integration and system tests on top of 100% unit test coverage.

However, you are not in a test maintenance phase, you are playing catch up. It's a lot better to have 100% of your classes under 50% test coverage than 50% of your classes under 100% test coverage, and your lead seems to be trying to get you to allocate your time accordingly. After you have that baseline, then the next step is usually pushing for 100% in files that are changed going forward.