This is a very important point. In fact, based on my experience today (trying to book flights for January), the Flex fares from MEL-SYD are exactly the same as the Business fares ($299 one way). That was the case on every flight I looked at. What's more, the discount code FSX46 knocks almost $30 off the Business fare but not the Flex fare, meaning the Business fare is actually cheaper!
For as long as this situation persists, the Platinum upgrade benefit is completely worthless. Even if you could make a small saving by booking a Flex fare and then getting the "free" upgrade, you would still only earn points and SCs at the Flex economy rate, plus you have the inconvenience and uncertainty of trying to find flights where the upgrade is likely to be available, and then having to phone up to book it. The Business fare actually allows more flexibility too (no fee if you cancel for a refund).
TBH, I'm not sure what the point of the Flex fare category in general is right now. I know Flex allows cancellation for a refund, but they still apply an $80 fee, plus you're paying a lot more to get that extra flexibility. As an example, there are multiple Choice fares available for $79 on the dates I'm looking at. If I book one of those fares, and I have to cancel, I will get a $79 credit. Even if I never used that credit, I would lose $79. Or I can pay $299 for a Flex fare, and lose $80 straight away if I want a cash refund. I really don't see why anyone would even consider choosing a Flex fare, unless nothing else was available.