Love the discourse re Merrily.
Noel, I think if you read what I wrote again you'll see that for me Sondheim does not entirely escape blame. I have never liked Good Thing Going, for example, for precisely the reasons you state - it's early 60s and that is NOT what a hit song from a show would sound like. I understand the levels Mr. Sondheim is going for, and yes, it's interesting to see the song evolve, but even Mr. Sondheim's own "hit" Send In The Clowns, is a "hit" I get musically.
But, I'm afraid, it's Furth who gets the lion's share and please remember I never saw the orginal, so I don't know how I would have felt about that - but in the revival he and Sondheim pretty much did what they wanted to do with no one acting as dramaturge - he basically told me that. And the book is just not good - a few lines of Furthian dialogue are funny, but it's melodramatic, arch, and finds no way to make us care about anything that happens.
I'm sure plenty of blame can be put on Prince for the original production, from the sound of it.