I agree. The main reason I signed up for Amex Platinum originally was to transfer to SQ. I got a good chunk of bonus points on signing up and an initial effective earn rate of 1.125 pts/$. Obviously bonus points are irrelevant as an existing holder, and the effective earn rate is only 0.75 pts/$. Plus SQ have devalued KF miles as well, so the value proposition has changed dramatically.
I'm not sure that you really can quite match that earn rate with a MC or Visa though, can you? The HSBC card earns 0.8 pts/$ but only on the first $3k of spend per month, then it halves. So if you're spending a lot, your earn rate would be lower than 0.75 overall. St George/Bank of Melbourne earns only 0.5 pts/$, Altitude Black and ANZ Rewards Black are both only 0.66. Am I missing something better?
In any case, it's unlikely to be possible to justify the Amex Platinum fee just to increase earn rate by 0.14 pts/$, so as discussed above, unless you are getting enough other value from the card to negate the fee, it's really not worth it anymore.
BTW, while the ability to transfer to various other airlines, including QF, is great on paper, I've never actually done that! QF points are very easy to earn elsewhere, so I've never felt the need to transfer from Amex. And as others have pointed out, the Everyday Rewards option now offers a backdoor to QF from much lower fee Amex cards.