I have heard many resons why St John XXIII called the Council but that is a new one on me. If you read the the Scriptures you won't find any evidence that God became man in or to make our "sojourn on earth less sorrowful." It was His concern for our life after this world that He became incarnate.
I was around for the last two decades of the 200 years you say the Church was "mired in sadness and triumphalism" and I never noticed anything of the sort. I don't know how old you are but you certainly can't speak with any experience of most of those 200 years. At least there was no doctrinal chaos like there is today and the Church's were full.