Is it not possible...

... to make a larger coil with a switchable "active" coil within the enclosure, that way you could run the stock (9x11DD) coil and switch off part of that coil to run like a 5" DD?

Or maybe the part of the coil that would be switched off would act as a nearby target.

Hmmm...

Garrett had those type coils in the 70's.

Cost prohibitive to companies as once you had a large coil would not buy a small one meaning less revenues for the companies is one aspect to look at as accessories provide extra revenue for companies, however is a good thought...

Due to the nature of the design, their effeciency factor is a bit reduced; hence, not desirable.