Over the past couple of years Williams has bounced back considerably from a period of relative irrelevancy following the tepid reception to his solo album In My Mind, and quite possibly the widespread realization that people don't really care at all about N*E*R*D records outside of their singles. It's an impressive comeback made all the more so by the fact that he hasn't substantially changed anything about his approach to music since the Neptunes became superstar producers around the turn of the millennium. He's still making beats out of cowbells, flattened drums, and vintage keyboard sounds, still singing almost exclusively in his trademark falsetto, and still sounding overall like a really abstract Marvin Gaye impression.
Pharrell's essentially a one-trick pony, but the key to his success is that it's an extremely good trick. And the pop audience is ravenous for it. The Neptunes dominated the musical zeitgeist during the first half of the aughts in a way that few artists ever have, and after being inescapable for eight years or so Pharrell's trick went out of style for just a couple of years before the pop audience decided to make him inescapable again. That's a remarkably fast turnaround.
And he's probably going to keep hanging around the pop landscape for a long time. The most notable names in the weird, Internet-addicted rap/R&B scene that bridges the mainstream and the underground have taken Pharrell as one of their spiritual-slash-stylistic leader. In the past couple years he's collaborated with Kendrick Lamar, Azalea Banks, Frank Ocean, Earl Sweatshirt, and Tyler the Creator. (Obviously he's a big deal with the Odd Future posse.)
These are the artists who are going to be influencing edgy, pop-oriented artists for years down the line, and they'll only serve to amplify Pharrell's own personal influence along the way. I feel like it would be safe to pencil in a Pharrell revival every five or six years for the next several decades.