I can see why you, as someone holding a ticket to see the show after Hank's departure, would be excited to see Robin Williams join the cast. For me - someone with no reasonable prospect of seeing the show again, at least not before the original cast is long gone - I would be (selfishly) a bit disappointed for the sake of Tim and David, whose star status might suddenly seem pretty marginalized. Of course I may just be projecting how *I'd* feel; maybe they'd be okay with it, or at least see it as a worthwhile sacrifice. At any rate, I have to admit I'm jealous of your opportunity to revisit the show, as I know there were probably a lot of things I missed that I'd be able to pick up on were I to see it again. Then again, given the difficulty and expense of getting seats now, I suppose I should be grateful I had the chance to see it once!
As for other appropriate replacements for Hank, I haven't a clue. I'm terrible at coming up with names for the"casting game"...although that doesn't necessarily stop me from criticizing the names suggested by others more imaginative than I.