You are both wrong. A new 100-400 with a built-in TC would be an excellent upgrade. The present 100-400 works well with a TC and a newer model with a built-in one would work even better. A built-in TC gives better results, is faster to implement, and does not require exposing inside the camera to as much dust.
Myself and many others would not mind paying the extra cost for such a feature.
How could I be wrong when I offered pros and cons for both sides of the suggestion?
As for not minding paying the extra, what if it was $3,999?