I suggested he wait until closer to the end of the year simply because he won’t need to buy as many points if he ends up taking a trip. From a logistical perspective, a mid year revamp of the awards program is highly unlikely. The greatest risk is if Marriott introduces another published elite level above platinum and restricts complimentary club lounge access to platinum and higher level members. That change would affect a lot of people, and I cannot see them making a mid year surprise announcement like that. It is more disruptive than raising the number of points needed for a free night.
They are going to merge the programs at some point after this year. Why would they change the existing program when it most likely won’t be around next year? They would have to retrain reservation and customer service employees twice. I don’t see them doing that.
OP has lifetime gold, and it would cost him $1600 to earn 20k points at most Marriott properties ($3200 at Residence Inn) plus any taxes and fees. I’m excluding any points earned from systemwide promotions, property specific promotions, rates that offer extra points (any rate that allows extra points at less than 1.25 cents cost including taxes would be worth it), check in bonus points from drawings or other promotional giveaways, and any points for customer service issues.
The only reasons I would buy the points now is if he is planning to stay at a Starwood property so he can get lifetime platinum and match it to SPG platinum or if he is planning to fly on United Airlines so he can get United Silver if he doesn’t have any status on United. In both of those cases, he would gain something of value by having platinum status right now as an infrequent traveler.