Sondheim revivals are just that REVIVALS, so I wouldn't necessarily expect them to run for a year or two as they have already been in NY and other towns.
Even in their original form, the idea you put forth that Webber, Disney shows and shows like Mamma Mia will run longer is the sad trend on Broadway but that does not mean they are BETTER show. It simply means that the theater audiences are now looking for the same kind of mindless drivel they see on TV and in the movies. Good, meaningful shows that make you think are in short supply and it's not only Sondheim shows that have short runs. Risky, experimental projects are being killed in short runs everywhere.
Sondheim is still a genius...unfortunately he is crying out in the wildnerness right now and I, for one, am sick of seeing the trend continue. It's very sad to watch good theater suffer this way. I hope it will be revived
"Life is a lesson in humility"