This announcement has me a little confused.
Stuxnet did not target the PLC's directly, this is simply not possible as a PLC does not have an operating system in the conventional sense, but instead can only execute a series of logic statements. Viruses are really not a concern for PLC manufacturers. There may be a risk of a DDoS, but i'm sure the various manufacturers are working continually to eliminate all such risks of this.
What stuxnet did, was spread between laptops used to program these devices, and look for the project files. it would then modify the logic statements in such a way as to cause damage (e.g spin the centrifuges to full speed, then immediately put the motors into reverse without first slowing down.)
So exactly what part of the stack are Kaspersky trying to replace? Their press release words it as if they are trying to replace the software within the PLC's, but this doesn't need replacing, it is secure enough as it is. The only part that needs replacing is the windows laptops used for programming. Surely it would be far easier to just switch to a stripped down version of linux, or QNX for these laptops?
And how exactly do they intend for their operating system to work with existing software? To do this will require a Windows virtual machine, or at least a partial re-implementation of the Windows API.