I think memory retention has more to do with simplicity / emotional consistency.
The new skill you learn may reinforce existing areas if its components are transferrable. Also, I see schemas as compression in the brain; eidetic memory shows how much some brains are capable of.
Forgetting is very good for clarification. I think your example suggests that your spelling in English hasn't been established deeply enough; I understand why you might have an adverse reaction to learning it though (German spelling is far more reasonable).
But if it has a finite capacity, when does it fill completely and with what? I guess it's entirely possible that only kids who are growing up can learn to use the extra limb and adults won't be able to.
For instance, I'm learning German and I can feel how is messing with my English, specially with the spelling (English is not my first language).