At first glance, its not very simple.
If, for example, you had a tube that a TC switched in and out of, when the TC switched out, it would suddenly be a long extension tube and you could only focus at short range. So, some other optics will have to switch in to compensate. I suspect that happens in the Canon lens.
Lenses with TC's that are built in are not common, but have appeared in the past, and, they are used for some Commercial TV lenses.
I suspect that any practical implementation of the idea for general usage would be difficult or impossible because of the need to matchup the glass that switched in to a individual lens. That would not preclude someone making a dedicated unit for a popular lens like the 70-200 f/2.8L. It might sell for $700, but some would find it very useful.
Why not start a kickstarter project to design and sell one??