I can understand delaying the fix in order to maximize the warranty period, but I'm more curious as to why you haven't determined *whether* you're going to get the fix (as opposed to when). That said, if you decide to get the fix you won't experience a difference in warranty terms based on the mileage you drive per year.
Not just equivalent, but better TDIs are readily available. I used cargurus to set an alert just so I can keep abreast of what's out there in our area (I'm in SoCal and have it set to a 500 mile radius, though). I can't tell you what the cutoff is going to be since that's a personal decision. I can tell you what I did, though:
I had a 2012 JSW with just over 120K miles on it (I turned it in right at my tolerance level for deferred DSG and timing belt maintenance). I bought it for approximately $19K. I turned it in for somewhere between $17-18K last year during the first month they were taking appointments. We leased a 2016 eGolf to get us through the transition period until they were selling 2015s again.
Last month, I brought home a 2015 SEL GSW with 65K miles for 15K. I was looking at a Golf for 11K but I passed on that after my wife reminded me we are expecting a son soon (in May) and with our daughter and lab we won't have a lot of room. I got my buddy who was looking for a new car into that Golf...turns out it also had the lighting, technology, and parking sensors but the dealership didn't know it! I wanted those packages but the only GSW with them I could find was 22K.
I feel competent to do a pre-purchase inspection and I have a VCDS (which you could borrow if you don't know/can't find anyone more local to you) to check for any lurking issues. I'd strongly encourage you to upgrade from your 2009 into a 2015. See how much you will get for your buyback. Then check cargurus for a car you are comfortable driving. Compare the two numbers
Buy the 2015 you find and schedule the 2009 for return ASAP before things get trickier. The only snag would be if you have a manual and you absolutely must have a manual, because those seem more difficult to come by. The 2015s will have a warranty to cover most of the big ticket concerns and you can visually checkout the wear and tear, but you'd have to really pick a clunker to get something that doesn't have less wear than your 2009.
So much technology, too. The 2015s have an upgraded drivetrain, KESSY, better infotainment, nicer seats, dual climate control, etc.
You could also see what you are going to get just for doing the fix but keeping the car, but I don't think that's going to get you the most bang for buck based on what I'm seeing the market do. Some people are talking about not doing the fix at all and not taking any money from VW, but that just seems like leaving money on the table unnecessarily.