Maybe everything that could be done has already been done, maybe it's the end of an era.
Not true in my opinion. It just looks this way, because:
1) Films of the sort of Prometheus need absurdly big budgets, which are financed by big studios, which are in turn owned by media conglomerates (20th Century Fox -> Fox Entertainment -> News Corporation).
2) These guys operate in the way of quarterly balance sheets, they're not going to hand out $100 million just like that, they need profit assurances which means that:
3) Films have to depend on time-proven formulas, stock characters and reliable stereotypes. That's what people understand and what brings in the money. As a result, creativity is stifled to a degree, at least in mainstream movies.
There are plenty of excellent films out there, they're just not in the mainstream.