I remember there was talk of the next version being more difficult to root compared to current versions, but from what we know of Android L so far I don't think that's true, at least not yet (unless I missed something).
Google may make stock Android, and by association, the versions of Android used by OEMs, more locked down to get more enterprise customers on board with using it, but will they put a stop to open ROMs? I highly doubt it. Google has always supported the Android ROM scene with its own Nexus phones. I'm sure due to the open source nature of Android it will always be possible to create an open rooted ROM even if stock does become locked down.
This article is nothing to do with the openness of Android though. It's about Google's OEM program. There's no evidence the OS itself will be any more locked down than Android normally is. I give it a week max before one of those watches is rooted by someone on xda.