Every time I see a comment like this, I think of
http://journal.neilgaiman.com/2009/05/entitlement-issues.htm... .
It should be an even exchange: you pay for the product as it is, not as you want it to be. Yes, when real life intervenes and the 12 novel epic doesn't get completed because the author dies (R.I.P. Robert Jordan) that is a bummer, but even then I find it very confusing when people go "I'd rather not even start reading until the entire thing is done" (and personally I think Sanderson's end to the Wheel of Time was very nicely done: https://en.wikipedia.org/wiki/The_Wheel_of_Time).
To the degree that the world of modern software and release early, release often have trained people to expect a constant treadmill of content I think that is a net negative: if you're worried about the author's pace stop paying and resume when they are done. "what's to keep him or any other author from walking away at that point?" sounds like you expect him to be chained to his desk until his output meets your expectations.
"As a consumer, it's bad enough when I'm left in this situation when there's been no money exchanged (such as a series cancelled with the plot mostly unresolved)." just sounds incredibly entitled.