Steve: "I must say, I have usually opto-isolated into a 297, just in case."

Makarand: "I have not used optpisolation in my circuit. But circuit works fine."

Whose design will work when the stepper motor burn out and is replaced with another brand?

Design risks "it tested fine" should, in my opinion, be illegal when the result of a malfunction is that something moves or spill over or such.
If the design is something where a malfunction will not have effects beyond the unit itself, I would still design instead of test, but there it is a judgement, not a requirement.

Erik