Doing so would offer the tiniest of differential Benefit to Plats. Allowing Plats to use it on ANY buyable fare as it was is the wisest tweak. Indeed would simply leave things as they were until Xmas. Golds have cheapest 3 buckets excluded.
Right now there are near zero differences.
As there are a TON more Golds than Plats, clearly the benefit being offered to the small number who fly a Lot is a wise compromise. They were happy with it until Xmas so if 10% of those who used it up to Xmas use it now, no big deal to the bottom line.
Even to cancel or change an award booking gives Plats no exemption over a RED Status member. INSANE.
Every other top tier airline plan on this planet has little cherries like that that are worth aiming for. It is called carrot and stick. 
I've earned Plat for about 5 years and will again this year on booked flights already but like many others will not bother next year, unless they re-visit a few useful perks.
Right NOW Row #3 is about the only thing of use to me that Golds do not get. They near pulled that too and backed off after loud screeching.
and if they do then it's not too long a wait for your booked flight.
