I don't understand an approach from a Platinum card cardholder to a "non-Amex" approach. What's about just switiching to a green or gold card with a lower annual fee.
You can do your calculation your own if they are worth applying or holding. But in that case you will get a lifetime period for using the MR points.
If completely leaving Amex and switching to Mastercard or VISA there is only a very short period to use them meaningful.
So my idea would be to switch to a not so expensive colour.
Or get a no-fee Everyday and keep the MR points forever... No point in paying fees for a card if you don't want/use the benefits. I personally love the Plat benefits, but would need some kind of retention to justify the high fee. Can't see any point to either Gold or Green...