CT can be either on or off. It is that simple. I see no negative to that.
What you are asking for can not be done with the existing key, it would require a different function to maintain what we also have today.
I in no way saying your wish is not a good idea.
But if you are a person who flies with CT off, Right now when you come out of auto pilot your plane is in trim.
Then later you can switch to CT on. Or if you did a manual trim, you can again hit CT and would be back to CT on.
Under your method if you turned CT on it would now be on when coming out of auto pilot Now press a trim key turns combat trim off, but pressing the key after that would also turn the default setting off instead of on and hence not work like it does now.
HiTech
Hence