- If they were fake (i.e., not real e-mails that were stolen), the people involved wouldn't be blaming the hack on the Russians. They'd be saying they were fake.
- Hillary Clinton actually addressed some of the issues brought up in one of the e-mails during the last debate (the excerpt from her Goldman Sachs speech in which she talked about saying different things in public and private). If the e-mails were fake, she'd just say so and not feel the need to justify what she was quoted as saying.
"Seems trivial to edit/create text files purporting to be stolen email..."
It would be a lot of work to create thousands (maybe tens of thousands by now) of fake e-mails that could plausibly come from John Podesta and the people he exchanged e-mails with. And unless the fakes were created by a group of insiders who knew these people and their dealings very well, there would inevitably be all sorts of errors that would make the fakes easy to disprove. (Remember that all these e-mails are published on Wikileaks' web site, and anyone can pick through them if they want to try disproving them.)