I have been hesitant to comment on this thread, as a admitidly former rent-head.
As much as I'd like to see Rent play for 18+ years on broadway, I am not sure if it will have the sustaining power.
A few variables will infuence how much longer it will last.
a) The success of Rent, the movie. I personally know I am going to love it, but will the general public have the same reaction to Rent as they did Chicago. If that does happen, Rent will continue to putter along, like Chicago has been for the past few years. I personally think the producers have been keeping Rent open because of the upcomming movie version of it. I think they would have closed it otherwise.
b) A friend of mine from out of town saw Rent for the first time this year. He thought it felt dated, already. Cats really wasn't "time-sensitive". Chicago is set long enough in the past, that it is nostalgic, same for Phantom.
I agree with EverythingIsRent, that Rent is a relatively inexpensive production, but it is still a Broadway show, and has to adhere to Equity contracts. So, it is a significant cost keep it running.
Also, does Rent attract the type of tourists that Phantom or Chicago does? As much as I don't like to admit it, NY Tourists is what keeps shows running for decades, not New Yorkers.
EverythingIsRent, I agree with you that the issues in Rent will not go away, but I am not sure having relevent issues can sustain a Broadway show alone. There have been many broadway flops about relevant issues.
I hope Rent can last as long as possible, but I don't think it can last another 10 years.