Voting is only meaningful to the extent that it achieves a given outcome. Now, you can argue that there's a form of protest in not voting (I certainly considered it this time around), but voting is one of those situations where inaction is nonetheless contributing to a specific outcome. In this case, Democratic supporters not turning out is one cause (among several) for Trump's victory. Tautological though it may seem, Clinton would have won if more people who supported her (or at least disliked her less) had voted for her. Thus their decision not to vote literally took votes out of Clinton's (theoretical) total, which in turn was one of the reasons that Trump had more. It also fails pretty badly as a protest. The apathy of the average voter is not a new narrative, and any idea of a protest vote will be swallowed up by the traditional one.