I'm hoping the battery pack design is long lived and in 10-15 years I can upgrade to a larger pack, even though realistically I should be just fine with a 40kwh battery.
Do you really expect to still be driving your car in 15 years? Have you ever had a 15-year-old car that was worth making major repairs/upgrades to? In 15 years you will want the newer technology, which will include features we have not yet thought of.
I'm not aware of any car manufacturer that has ever offered to upgrade older cars with newer technology, though they all offer replacement parts for a decade or so. The Roadster pre-paid battery replacement plan merely promised a replacement pack as good as the original. It does not promise a newer battery type, though Tesla could choose to do that if they found it in their interest. Basically, there will be replacement batteries available, just as you can get a new clutch or transmission for your stinker. If you have bought the plan, you've already paid for it. Otherwise, you pay cash.
Nissan told me, way back when I was reserving the Leaf they never delivered, that there would be an optional battery upgrade later, to a longer range, but I regard that as more of the same dishonesty they bombarded me with throughout my futile wait for the car.
Perhaps (???) you'll be able to upgrade a 45 or 65 kWh Model S to one of the larger packs. But I highly doubt you'll ever be able to upgrade beyond what's available new today. You will definitely be able to replace your pack with an equivalent one, probably for a decade or so. At some point they'll stop making parts, as every car maker does at some point. Fifteen years sounds to me like a long time to make parts for any car, though there will be used parts at junk yards. (Though how long a battery pack lasts in a junk yard before it "bricks" is another matter.)