The OP is experiencing some anxiety over the possibility that Marriott may change the rewards program. Right now, there isn’t much difference between gold and platinum at Marriott. That may change, but I doubt it will. Even if it does change, it won’t happen until next year at the earliest.
The OP should wait until near the end of the year to buy points. He may have some travel in the next few months, and it would reduce the number of points needed for purchase. Marriott will not make any large changes to their program without giving some advance notice.