But that's my point. Why wasn't it left retired back in the 90's? What bought it back?
I mean I absolutely love the X-Files, but would it last on today's TV only 10 years after it went off air? I'd say a resounding no.
It was retired in 1989. After years of hope there was the movie in 1996. The fans and DVDs brought it back.
Stepping away from the series there have been remakes, returns and attempted remakes of many other things - the Prisoner, Randall and Hopkirk, Red Dwarf, Blake's 7, Battlestar Galactica and V. I'm not sure how often the tv series should continue, but really it is in the context of many other things. It seems common for many new series particularly in the United states to either get axed or end up running for 7 seasons. What you say may be true of doctor who, but it isn't just a doctor who thing.
A lot of retro series that were shot on film (mostly in the US where things were done differently) have been scanned into HD from film and play around with and released on blu-ray and then tv channels have become interested in rebroadcasting them for the nth time. In future what will probably matter in terms of revenue generated will be whether the current Doctor Who was shot in HD or not. The Easter special in 2009 was the first to be in HD. Sadly in the long run that probably means Doctor Who will generate slightly less money in future than it might do otherwise.