George Lucas is not out hawking outdated technology. He's selling entertainment. You get two big hours of entertainment from a big movie on the big screen with big sound in a big theater, for a small $8. In fact, it only cost me $5.25.
It doesn't matter if the movie is 20 years old. You can watch it again and enjoy it again. Even if the movie is the same, you'll probably have a different reaction to it, since you're now 20 years older and wiser. It's just like buying and re-reading an old book, seeing the same koala bears in the zoo again, or going for another ride on the roller coaster (which, by the way, all cost more now than they did 20 years ago, even if it's the same bear sitting in the same fake eucalyptus tree).
Moreover, most of today's college students weren't even born in 1977, when Star Wars was first released. These people would be paying for a completely new experience, one which doesn't compare at all to buying MS-DOS. You can enjoy a 20-year old movie that you've never seen before, but what are first-timers going to do with MS-DOS in 1997?
It's true that George Lucas is making a lot of new money with very little new investment, but he has the supply and there is a demand. Microsoft bought MS-DOS from a small company for a paltry sum, but they were able to create a huge demand for it and came out with staggering profits. It's okay when Microsoft does it, but not when George Lucas does it?
So, the biggest difference is that people want to see Star Wars again, but nobody wants to hassle with MS-DOS again. Everyone is of course entitled to their opinions, and if you don't want to watch Star Wars, that's fine. "To each his own," as Robert himself loves to say.