Obviously it's feasible - the question is, does the market opportunity justify the investment, given what the cost and size/weight of the lens would end up being? What you call "a lot" of people buying Canon/Nikon lenses and speed boosters with them, is actually a TINY amount relative to the numbers needed to justify an entirely new lens design. The 12-40 f2.8 is already about as heavy a lens as most folks wish to put on their camera, and a 12-50mm f2 would be at least double that weight, using simple physics of comparing f2 aperture glass to f2.8, and also for a longer length lens. Which already limits it's market, which means it would need to cost quite a bit more than the 12-40mm f2.8, which limits it's market some more.

I don't think it would be 800 pounds - That's what the existing 12-40mm f2.8 costs - and it's half the mass, and made in much greater volumes. I DO think it would weigh around 2 pounds at a minimum. And no, I would not be interested. I can already get a 12mm f2 that's small and light, and a 45mm f1.8 that's small and light, and a 25 or 17mm f1.8 lens that's small and light. I would not prefer a zoom lens that weighs more than all 4 of those primes combined, and costs about the same as all 4 of those primes combined.