When I create my own workouts, there's a fairly doable workaround: you select "distance" when you first open the workout builder, then you change each block to "duration." It will continue to calculate overall distance for the workout.

But, this is a giant limitation for purchased training plans. In a half-marathon plan that I recently purchased, all the workouts were created with "duration" as the default which means I can't see my estimated planned distance by week.

I would really like a solution that either defaults to estimating planned distance for all workouts (preferred) OR where toggling "Estimate planned values" on and off would recalculate *all* the estimated/planned values (incl. distance), regardless of whether the plan is based on distance or duration.

It's doable. speed=distance/time. Once you know 2 of them, the 3rd one (no matter which one) it's easy to calculate. In fact there is trick to do it at trainingpeaks, but it should be done automatically, no tricks.