S550 and CD4 are already using the same production line so my guess is there is a pretty high degree of workflow alignment and parts sharing going on.
The next gen which looks like it will be a heavy facelift of S550 and CD6 are probably on the same level of mutual compatibility even if they are not identical.
You don't need to be one the same platform or architecture if you can get most of the value and benefit out of the assembly side and procurement side anyway. Like how Ford Europe and Asia have been able to build C1, C2, B1, EUCD, and CD4 vehicles all on the same assembly line. As long as next gen Mustang can get use the same drivetain package as CD6 and be assembled in an existing facility without a lot of new tooling, it stands to reason that you don't necessarily need to move platform just for the sake of commonality (that you already can achieve in practice anyway)
The pay off on the common platform and architecture is if you need to redesign the vehicle very frequently or need to spawn a lot of derivatives. The common hard points means you can model and simulate a lot of crash safety and vehicle dynamics with high degree of accuracy so it cuts down on development costs. It also make drivetrain and other engineering less complex because you design to fit one vehicle, it will fit in ALL vehicles. Mustang is on a pretty long model cycle and Ford is not planning to build other vehicles off the Mustang platform so the need to move to CD6 is minimized. If Ford was going to put Mustang on a hard 5 year model cycle with a 3 year facelift then it pays to be on a common platform. As is, Mustang seems to be doing ok on a 5 year facelift and 10 year model cycle between complete redesigns.
In theory, you want volume models like Focus and Escape to be a common platform so there is always a fresh version of the platform going into a production vehicle each year. That helps with cost amortization and continuous improvements. Minor improvements made for one model and be quickly added to others during midcycle facelift or redesign without breaking the bank. There is no reason for someone like Ford to redesign steering rack, HVAC system, or suspension on volume models everytime they redesign the car (but that was apparently what Ford was doing until fairly recenlty).