Usability should include accessibility when possible, but there is no must here. There are lots of tools that need usability testing that have a sufficiently sm

Usability should include accessibility when possible, but there is no must here. There are lots of tools that need usability testing that have a sufficiently small team and user-base to make accessibility a waste of time and resources. Indeed, on teams and products of any size there is always a decision to be made about which accessibility requirements will be met and to what degree. I would make the separate argument that what you need with accessibility requirements is separate testing expirements for each one. You don't need 100 testers to to make sure you have some special accessibility requirements, e.g. you need 15 normal vision testers and 15 impared vision testers.