Established markets still need PMF. And existing sites/apps can lose their PMF.
There's a large demand for, say, a more privacy based social networking website. Or a gaming based social networking site, like what FB-Zynga used to be.
Google has a large blind spot too. Yelp is filling one, Pinterest covers their image search weakness, tons of travel apps/sites are filling another. But there's many, many things that can be more localized or made into a niche.
I'm not sure if it is harder, as technology has significantly improved. I think we're at the stage where less of the effort is building/designing an app, and more of it is in better product development.