Shouldn't a writer, especially one of a multi-million dollar project, be one who does writing for a living?
No.
Exactly, and besides, it isn't as if Simon Pegg and Doug Jung haven't written before or know how to handle the deadline. In fact, Simon Pegg was already known for some of his work before the 2009 film which were well received (Shawn of the Dead, Hot Fuzz, etc.) Doug Jung has also been a writer for a couple of TV series too. So I'm pretty sure they can handle the deadlines.
It's interesting that, at least on a critical response level, Pegg & Jung's work (albeit they never worked together until now, that I know of) perhaps have a more favorable track record going into this film than Orci & Kurtzman had going into the 09 film. That's not a comparison by how I view their works, by the way, but going off of reviews, ratings, etc. Obviously Orci & Kurtzman had the box office track record advantage though.
I'm excited at the prospect and perspective Pegg & Jung could bring to the table.