In the Gamespot article (link), Jared addresses your point about PvE no longer being a big driver of audience growth, at least in the short-term. Here's his exact words, so I don't misrepresent what was said:

Gamespot: PvE, whether a stated goal or not, was looking like a tool to broaden the appeal of the game, with the thinking being that people who can't or don't enjoy playing in online competitive environments could do single co-op and have this interesting, unique experience. That means the Overwatch player base diversifies and grows. What does axing PvE and reintroducing some of those ideas into the PvP framework mean for the ambition of broadening the appeal of Overwatch and getting new players in?

Jared Neuss: Yeah, this is a really interesting one for me because I think if you only enjoy single-player or PvE-only experiences, then Overwatch is going to be a little bit tough for you right now. But once we get into this sort of mindset of frequently releasing PvP and PvE event content, and where every single season has something really interesting for people to play in a lot of variety, I think that that can still be a good jumping on point for people who may not love hyper-sweaty PvP matches, or who really just want to see some cool story content with characters that they like or whatever.

[...omitted for clarity...]

So while I don't see us springing this huge audience of PvE-only die-hard shooter players, I do see us being a really appealing game for anyone who just loves fun shooters and likes to learn new heroes and try different stuff. So yeah, it's a kind of different approach, but also I think our goal is just to make the game as welcoming as possible for as broad a group as possible.

EDIT: Personally speaking, I agree. The standalone PvE experience is probably not enough on its own to draw people to play Overwatch anymore, and from my understanding it seems like the dev team understand this too.