With the caveat that we're all guessing right now, I would sincerely hope that any and all Starwood Platinums are reflected as a minimum of Marriott Platinum on August 1st. If you have legit SPG Plat through 1/31, then you should have legit Marriott Plat in the new system through 1/31.

My understanding was that it's the people who matched from SPG Gold to Marriott Gold with 10-ish stays only in the Starwood system who will be left out in the cold during the merger.

If there's something wonky that matching a SPG Plat to Marriott Plat somehow wipes out the system's view of a legit 50 EQN year in the Marriott system, that would be really poor IT on Marriott's part. But we have no real indication that this will happen.