If you fly enough on WS (to be Platinum) to use the credit for a changed flight in the future, why not buy non-Flex Plus, be assured of a seat and at worst you change or cancel and use the credit later? There's no guarantee at T-48 that there will pay for upgrades left to pay for.
If they are doing silly random decisions on if and then who for "free" don't play along. If you want Plus, buy it and then it's assured vs. a game.