I'm not a Plat card holder, but a Premier Gold card... and yea, the Chase Sapphire card has pretty much supplanted my Amex (as I've detailed in other threads previously).

I'm in the same boat as you, in that CO (and BA) were pretty much the only 2 partners I use to redeem my MR points for ... with one leaving, and the other facing a major devaluation, becoming very unlikely I will stick around. I have other Amex cards like SPG and Hilton that I'll keep, but I'll be done with the MR program until such time that they add other, meaningful partner redemption options.