I think this is the biggest downside... I believe these challenges are one time only. If you use your challenge now, you may end up wanting it in the future if you take a year off for traveling for some reason.

So, assuming you'll stay Gold this year regardless, the question will be whether or not you can earn 75k on the challenge, and if so, if having 75k instead of Gold for 17 months will be worth having used your one challenge.