traditional films are DESIGNED to look like they have depth and 3D in most cases, just as seen through a 2D plane. That's why they so frequently layer things so much in the composition, and use dynamic lighting to make images pop.
I'm thinking any practical 3D conversion process would rely on that to some degree, so the computer can estimate just where everything should be.
Personally, I like classic 3D movies. Anaglyph (red/blue) doesn't work particularly well, and it barely works on a colour film. I've only seen "Up" with the polarization process they use now, and it was pretty impressive. I think 3D is great, as long as the movie doesn't try to ride on visuals alone.
I wouldn't want to see non-3D movies converted for the same reason I don't like colorization. The movie wasn't made that way, and if it's good enough to be worth converting, people shouldn't need colour or 3D to enjoy it. In fact, many obviously don't need it, and the ones who do are just as likely to turn their noses up at an old movie no matter what it looks like.
I don't think 3D is a fad. It's here to stay for a couple of reasons. The technology has come of age. CGI can be just as easily rendered for both eyes, and the new filmless production methods make it more practical for live action. And with today's render farms, the extra time to do things in 3D is going to become more and more negligible. The cost-to-benefit ratio has changed dramatically.
The other, more important reason is that theatres need 3D to keep ahead of home video. With relatively little money, you can project a picture the size of your rec room wall with crystal clarity, you can have surround sound with bone-rattling bass, plus you can save a bundle of money in the long run, watch anything you want, and you can pause for bathroom breaks. The only things the cinema have left are the really huge screen, the experience of watching with a crowd, and the fun of a night out. They need 3D, as something that is a long way from being affordable at home, and so far uses a technique better than anything you can get at home.
It's pretty clear just from the money theatres have spent on 3D projection equipment that 3D is no longer a passing fad, as it was in the 50s and 80s. We're going to see more of it at the movies, and it's really the only direction left to go for the development of home video. We're gradually going to see home systems that are cheaper, more realistic and less cumbersome than the field-sequential systems that are currently the state of the art at home. Won't be long before people are paying an arm and a leg to install scaled-down polarization systems in their homes.