The only person whose opinion 20% time, as prescribed, does not value, is your manager's. (As they shouldn't be able to veto it.) Your 20% project is not going to get anything done if you're not taking the opinions of your collaborators/dependencies/customers into account.
IMO, The reason most people don't pursue 20% projects is because it's hard to be the manager, product manager, and main developer on a project that you're giving 1 day a week to. It's really, really hard. Most people don't have the skillset to do that well.
So the path of least resistance to a simple life is to just devote 100% of your time to what your manager wants you to work on, where its her job to manage, product manage/wrangle product managers, etc.
IMO, The reason most people don't pursue 20% projects is because it's hard to be the manager, product manager, and main developer on a project that you're giving 1 day a week to. It's really, really hard. Most people don't have the skillset to do that well.
So the path of least resistance to a simple life is to just devote 100% of your time to what your manager wants you to work on, where its her job to manage, product manage/wrangle product managers, etc.