> The solution is to use an A/B test running time calculator to determine if you have the required statistical power to run your experiment and for how long you should run your experiment.

Wouldn't it be better to have an A/B testing system that just counts how many users have been in each assignment group and end when you have the required statistical power?

Time just seems like a stand in for "that should be enough", when in reality you might have a change in how many users get exposed that differs from your expectations.