There is the same disincentive to book away from CO. You either want to hedge your bets or you don't. If you want to hedge your bets, then you will stop with CO at 75k - anyone that will run over the DM threshold could earn mid-tier with someone else, along with CO Plat. With DL, there is some temporal hedging going on but, as I stated, this should only be expected to encourage you to shift flying forward.

If you don't care about hedging your bets and simply want to maximize your utilization of benefits, then you should remain with your preferred program for all flying. There is little reason to stop short with DL to enable status for another year, only to then not travel with them. At best, such a strategy yields you little more than a soft-landing program.