Happens sooner than you think. Look at producers like Bruckheimer, riding the coattails of Jack Sparrow for the past decade. He has done NOTHING of substance on the big screen that I'm aware.
Then you go to the Emmerich's, who can still produce modest hits, but not for those massive budgets anymore. Jamie Foxx and Tatum in a White House siege movie? What planet are you directing films on? Oh yeah... Independence Day 2.
Spielberg hasn't been relevant in a long time. Some nice niche films like Lincoln, but its few and far between. They had their runs. I really think an era has come and gone. It's a handful of guys now, starting with Nolan, at least in terms of mainstream. I don't think anyone from a pure director standpoint matches Nolan in terms of big budget films, aside from Cameron who doesn't direct enough for me to consider him. The other top guys these days are too artsy and can't get the massive budgets for many of the projects they choose to direct.