> As a community of mostly engineering-minded folks, I hope we can take this as a warning to not make our mission-critical systems depend on inputs that are merely convenient side-effects of third-party systems provided free of charge and without obligation.
That is not entirely true. The TSO is committed to keeping the average frequency of the power grid at 50Hz and has always communicated that. It tries to keep grid time within 20 seconds of UTC, and has historically achieved this. So it is reasonable to use the grid as a time source where high accuracy is not required. The current situation is highly irregular. Using the grid frequency for time keeping is a secondary function of the electricity grid, but it is not merely an unintended side-effect.
I agree with the rest of your points about using a different time source where high accuracy is required.
That is not entirely true. The TSO is committed to keeping the average frequency of the power grid at 50Hz and has always communicated that. It tries to keep grid time within 20 seconds of UTC, and has historically achieved this. So it is reasonable to use the grid as a time source where high accuracy is not required. The current situation is highly irregular. Using the grid frequency for time keeping is a secondary function of the electricity grid, but it is not merely an unintended side-effect.
I agree with the rest of your points about using a different time source where high accuracy is required.