As you quoted, the one time payment for Pro is a payment to purchase a copy of the program...exactly that You pay your $18.88 USD & you've bought your version of Pro. You can keep using that, (plus any updated versions that you download during the 6 months) until the cows come home!

As it says on the Pro page, your payment covers you for up to 6 months worth of free updates...so therefore after the 6 months are up the updates aren't free!!
You don't have to update, you can stick to whatever versions you've downloaded but if you choose to update you'll have to pay again

You won't be charged again if you decide not to update. The version(s) you've bought are yours. But...it's every user's responsibility to save & back up their installers. People who don't bother to save their installers & then decide they want to use a version they'd previously downloaded will find they have to buy again to get more Pro. Which is fair enough really...if you buy a program from a shop & lose the disc, you can't go back to the shop & get another copy for free

I hope this clears things up a bit. You don't have to update, it's personal choice whether you do or don't. But you do have to take care of your installers.