I like Sondheim...I'm not a rabid fan like some, but I just feel like I have to respond to your post, point by point.
1) More people seem to be liking The Frogs than not. I don't think there is any clear indication that it is not going well, and by the accounts on All That Chat this morning, the addition of Roger Bart and his chemistry with Nathan Lane, has also made a difference.
2) Bounce never did make it to Broadway. I can't say it's my favorite Sondheim score, but there are some nice items in it, and it's not to be wholeheartedly dismissed. And considering in his nearly 50 year career, it's Sondheim's only road closer, it's not such a black mark on his name. Certainly, his outright flops, Anyone Can Whistle and Merrily We Roll Along, keep coming back again and again. I can't imagine we've seen the last of Bounce either.
3) Assassins won five Tony Awards, and considerable critical acclaim, a coup of sorts, since the show was so roundly dismissed in its first New York incarnation. Passion won four Tony Awards and was also hailed by critics, though it sharply divided audiences into "love it" "hate it" camps. Neither could have been expected to have had long runs, given that the material for both shows is often so dark and uncompromising, but faulting them for having shorter runs than Sunday or Into the Woods is beside the point. They couldn't have been expected to run longer than those other shows. Even Sunday in the Park with George is more accessible for a general audience than Assassins or Passion.
4) The Frogs has never had a mainstream Broadway production. What's so wrong about him revisiting it and writing six new songs for it? At the age of 74, and with 16 Broadway shows to his credit, is the man not allowed to slow down a little?
To call him "finished" is an argument that the young should be eaten.
"Gif me the cobra jool!"