While part of me understands this "I've made platinum for the year (or the rest of my life, for the emeritus folks), so now I'll work on earning status with someone else" mindset, when I deconstruct it, it really doesn't make sense.

Is the reason you've stayed with Marriott so much simply to get "recognized" for it? I suspect that for most of us it's because we know Marriott offers a superior product. After making platinum you could spend your next 25 nights in a Hilton, but why? You'll be treated better, have a nicer facility, and earn more Marriott points (including the platinum bonus) by just sticking with them. Unless you just like to show off an array of elite-tier cards, the logical thing to do is to continue patronizing the brand that has earned your loyalty through the superiority of their product.