DOOM is going to be in use for a long time, but just this year, hardware has surpassed a really significant point with the floating point pixel formats and generalized dependent texture reads. These are things that demand that a new engine is written.
[snip]
With game development taking multiple years and costing many millions of dollars. I think real innovation will not necessarily come from triple-A titles. Triple-A titles have so much of an investment that it engenders a huge risk-aversion. I think that the real innovation will come from things that are done on a smaller budget, that might be targeted as budget titles. It's okay to risk $500,000 on a development budget where you might say, "This is a clever idea, lets give it a try." Risking $10 million dollars, and some development budgets are going well over that already, is a lot harder.