Your statement is wrong. Marriott representatives do state that currently earned Platinum status is going to be valid through above mentioned date. Of course you may go on arguing that they can misinform people, but I doubt that could be the case.

Marriott is not the only player around, and if they drop even their own elites, they will simply lose previously loyal customers. I am indeed unsure if status earned in SPG will be recognized equally, though I tend to think it also will.