Reeder is an app I bought years ago, that my wife and I both use daily, and comprises a large percentage of our time on our iPhones and ipad. I got my $5 worth after 6 months, and I'm happy to pay again to ensure that development continues.
There's something very pernicious in this story. The fact that you almost feel obligated to give this developer money just so that he doesn't stop supporting the app seems disconcerting. Developers have an obligation to support their paying customers even if it is not expedient for them to do so. I would never give money to an app simply out of fear that the developer is going to cease developing it; that just seems like throwing fuel on the fire.
I'm not disputing anything else you've said, but how did you decide that you'd got your $5 after 6 months? I wouldn't have the first clue how to go about deciding at which point in time I got my money's worth.