It must be optional. Some trainers don’t need calibration each ride (many wheel-off trainers), while others do (most/all wheel-on trainers).
And the Tacx Neo1/2 don’t need any calibration at all. So there would need to be great intelligence in the programming based on the trainer in use. But I see it as easier to leave the calibration as “optional” but flagged with a button.
Let the user decide as needed. There are reasons that people may want or need to skip calibration (already done in a prior workout, or other desire to skip), so a forced one could be annoying. Maybe not bad if it was easy to skip, but I see it being “a signaled optional” and manually selected as the better option.