1. I wasn't aware that Mooney was still selling the M20J . . . must have missed that in their product lineup which mainly focuses on trying to sell $700K+ Ovations/Ultras. If you had tooling and processes down to a science after having made 10,000 M20Js for a few decades, you think there's any R&D/tooling/depreciation that is still having to be absorbed? God I'd hope not. So you're left with the cost of the raw materials and the labor to assemble. I really hope we're not saying that a M20J has labor/materials over worth over $150K (assuming non-glass panel).
2. Vans seems to be doing just fine making a profit on RV10 kits at $50K/pop. I can't imagine there's a ton more cost to build an M20J/C172/PA28 in terms of materials (especially with volume discounts). So call it $50K in materials, $50K for engine, $10K panel. $110K is about what you can build an RV-10 for, from what I've gathered, if you don't go crazy on panels/upgrades. So, Cessna has to add the value of labor into the mix. Call it $30K if you want. So we're around $150K in cost to build. You're telling me that Cessna has to turn around and sell that for 50%+ margins to make the numbers work? If you allocate the cost of all of Cessna's overhead expenses to a product line like the C172/C182, it's going to drive the price through the roof.
I'm not talking about expanding the market, per say. I made no implication that if Cessna/Piper just dropped their prices, thousands of people would come running to the table with money in hand. I'm saying that the pricing of current aircraft like the long-established C172/PA28 series ensures that no one but revenue-generating businesses (flight schools) are going to be purchasing new. Hell, lump the straight-legged C182 into the mix, since aside from the engine, there really shouldn't be an appreciable difference in price . . . you think there wouldn't be some people/flying clubs lining up to buy a sub $250K NEW 182?! I bet there would. Cessna would have to accept making 30% margins instead of 50%+, which isn't likely going to happen. As has been mentioned, Cessna/Piper has no interest in investing anything into the legacy SE aircraft, there's just no justification for them when they can earn more profit from the sale of a few Jet-A burners than total annual sales of the C172.
All in all, unless you can see Mooney/Piper/Cessna's internal books, you and I are just speculating anyway. We're just operating under different assumptions, and I assume that it shouldn't cost a major manufacturer a ton of labor to make a product they've been making for the better part of a century.