I didn't express any direct or detailed criticism of at-will in my comment, just my gladness that we don't have it.

Why, despite the difficulties it causes me when dealing with poor performing devs, am I happy we don't have at-will?

Because at-will dramatically distorts the employer - employee social contract in favour of the employer. And employers already have a disparately large amount of power over their employees as it is. So there you go, that's my criticism of at-will employment. It puts the well being of the employer ahead of the well being of the employee.

You can have a trial period without "at-will".