Exactly my dilemma!
Not sure I'm going to be much help, but when I read this I thought to myself that is exactly my dilemma. Have been Diamond/Platinum the last 5 years, and again Platinum for 2018 but with only 10K rollover miles. So with the 10K rollover and 20K MQM from the Amex Platinum card for '18, I'm at 30K. My business travel will be greatly reduced this year, and my company is forcing more folks to lower cost options, so I don't expect more than 15K MQM on business travel. Leisure travel will be one trip to Europe so that is 8K MQM. So if it all comes together, I can make GM for 2019 by a hair.
I have also been thinking of just dumping the Delta Amex for the Chase Sapphire reserve which I also carry. It's a 50K spend requirement on the Amex to hit the 20K MQM, which is $750 in travel credit if I use them with Chase. I save the $195 annual fee, but lose the companion cert which I do use, and at least 70K in non MQM miles (50K spend miles and 20K bonus miles). So for now, that still seems to outweigh switchin all spending to Chase.
So my plan for 2018 is to split my spending and put just enough dollars on the Amex card to hit the 20K MQM's and make GM for 2019 year. Then put everything else on the Chase card. Another factor for me is I'm at 850K million miler miles, so hitting 1M would be nice to be at least a silver for life, so it still make sense to fly Delta for a while. Anyway, not sure there is a perfect answer and I am also interested in other perspectives on this. And I pay all my credit card charges off every month so interest rate fluctuation doesn't matter. Thanks.