PMax has taken over my brand shopping traffic and now claims the credit

Your reading is correct and the arithmetic you did is the right one - the fall in the shopping campaign matching the rise in PMax is as close to proof as this ever gets. On keeping it off brand: brand exclusions are the mechanism and they're partial. They handle the obvious cases and leak on variants, misspellings and navigational phrasings that don't literally contain the brand. Expect improvement, not a clean separation, and check what is still getting through after a fortnight rather than assuming it worked. On the argument for leaving it: there's one, and it is weaker than it sounds. If PMax is genuinely defending brand searches you would otherwise lose to a competitor, that has value. Test it rather than assume it - look at whether your brand impression share was under pressure before PMax launched. If nobody was attacking your brand terms, PMax is not defending anything, it is charging you for customers who typed your name. The thing I would push back on hardest is the reporting consequence, which is worse than the money. While brand sits inside PMax you cannot evaluate PMax at all, because its headline number is dominated by traffic that would have converted anyway. Separate it and the number gets uglier and starts meaning something.