Actually the OP raises a valid issue - wouldn't it make more sense to offer a Gold challenge & one has to earn Platinum? Or - whether it be Gold or Platinum, what about not offering any of the bennies until the challenge has been met? Either would satisfy those that are concerned about 'their' bennies being infringed upon by others.
I'm still of the mindset that for those who take Marriott up on the challenge there are others that won't, and it all evens out in the end. I also agree w/ SacTownGuy that I don't think most people sign up for a challenge just for a vacay stay. Sure there are some, but my guess is those who go for the challenge plan on staying at Marriotts.
And Marriott has decided that offering the challenge - vs just granting status based on status with another chain - makes the most sense. And by offering the bennies immediately, helps cement them to Marriott.
I do disagree w/ the OP's premise that the gold/plat challenges are responsible for people questioning the value of Platinum. There are bigger issues w/ Marriott that make people wonder that
Cheers.