Thank you @Nevin. As a meta commentary, the Testing Workgroup has recently been established, and many members of our group are new to participating in the Swift Evolution process at this level. Many of us have offered our thoughts in other SE reviews, often passionately, but now we are operating in a new capacity and it’s an adjustment.
I agree with the spirit of your critique, and think we need to stay mindful of this in the future — especially when a proposal author is a member of the workgroup. All authors should take great care to maintain an open stance during a review and be cautious about repeatedly defending their proposal, but this can be hard since they often hold strong views about the subject! I do feel @grynspan's comments have maintained a professional tone throughout this review, FWIW.
I think it is perfectly reasonable and expected that a proposal author participate in the discussion and clarify things, and even explain rationale, but there’s a balance that needs to be struck and sometimes it’s best to withhold responses after a certain point and let the discussion play out among other community members. I will make sure that topic is discussed in the next workgroup meeting. Also, I’m not sure this has been stated elsewhere but FYI: In the meeting where we decided to review this proposal, @grynspan proactively volunteered to recuse himself from the final decision since he's the author. This is a precedent which will be applied to future situations when a proposal author is a workgroup member.