Mmm. I dont think ppl are that much bothered about the fact that christians used to die in the arena, really. I mean, a film like Gladiator was still a massive hit, wasn't it?
I think that at the end of the day it's wrong to idealise the 'golden era'. Back in the 70s and 80s, the choice of sets was still very market driven. That's why there was the western theme, because cowboys and stuff were still very popular among kids our age (Im 35). Same goes for Space and what have you.
I agree themes could last for longer ... but then again I think it has to do with decreasing attention spans. Attention spans are diminishing by the minute, in all forms of entertainment. I mean ... if you take a look around you, teleserials are nowadays more popular than feature-length films, partly because they are very well-made but also because they are shorter. Id rather watch a 30 minute show nowadays, than a 90 minute film. I think it has something to do with why a theme seems to last only one year, too. In a roundabout way perhaps, but I still think there is a link.