What if there were no more new speaker models?

 "Why is there demand for newer speaker models? What is it that's being done that makes the speakers better than they were last year? Does anyone know?"

Vance Packard's The Waste Makers (1960) is a classic treatment of planned obsolescence and manufactured demand. Packard distinguishes functional obsolescence (it breaks) from psychological obsolescence (you're made to feel it's outdated). The audio industry it more inclined to the latter.

John Kenneth Galbraith's The Affluent Society (1958) argues that in mature consumer economies, demand is no longer discovered but created by producers. The want precedes the product, not the other way around. His concept of the "dependence effect" maps directly onto the OP's question.

Further back, Thorstein Veblen's The Theory of the Leisure Class (1899) addresses conspicuous consumption and the signaling function of goods. High-end audio is a nearly perfect Veblen domain: the price is part of the product.

The OP is asking the right question: is the improvement engineering-driven or market-driven? Galbraith gives the best answer — in mature industries, those two things have largely switched places.

It's all about keeping the ball rolling. If your marketing dept. is good, you have a shot at success. It's funny the OP used the analogy of the toaster. My mind went immediately to ROASTERS. I'm talking the boardwalk type you used to see as a kid. They built them so well, maintenance was at a minimum. They didn't know about built in obsolesce then. Guys are still using the same roasters today.

@jnovak - I've had the same Sharp microwave for 35 years that I still use today, and it's needed no maintenance, except for the occasional whack on the side to get the turntable moving.... 

Very interesting concept. Here’s a related thought: Are things truly better designed & made than years ago? In some ways yes & some ways no. Is a refrigerator w/ no Freon but electrically efficient ( many now use propane) that dies & winds up in a landfill in less than 10 years truly overall better for  our environment than one w/ Freon but lasts 20 years +? Same could be questioned for lead in gasoline & paint. 
 

If we go back to the 1960’s & had but a few loudspeaker choices & never had any more, we still could all enjoy listening to reproduced music at home today. Altec A7, original Quads or the AR 3A powered by a Mac, Marantz or HK etc amp w/ a Thorens, Dual, AR etc turntables w/ a Shurej V15 cartridge covered a lot of ground for a wide variety of tastes. 

do newer speakers have a longer shelf life. The soft material on the drivers from the 1970s degraded. I'm not sure what the current model speakers offer in the way of degradation issues around the drivers. I've lost some really nice speakers from the 1970s because of the driver degradation where that circular spongy thing decayed.