The $15,000 question: Why did Leica not make an interchangeable Leica Q variant?

$15,000 because that's the cost of the SL + kit lens.

But honestly, which of you wouldn't have wanted a Q body allowing the use of leica lenses, and being mirrorless, everything other lens mount out there? Heck, Leica could've gone up $500 for the option of a interchangeability (knowing them though, it'd probably be $2K extra just for this added feature), and these things would've been selling better than waffles at breakfast.