I do believe that light bulb manufacturers purposely reduce the lifespan of their products in order to generate more capital gain for their companies. With the technology that we posse,s products like light bulbs should be able to last much longer than the versions available today on our store shelves do.
As most corporations are in competition for profit and growth, the manufacturers of light bulbs want to ensured that the sales and revenue of their product have a faster turnover. So the shorter lifespan of a light bulb forces more frequent purchases. This reduced lifespan may be difficult to prove but it is all part of the corporate agenda.
Imagine you wouldn't have to buy all the stuff you own over and over again because it lasts longer. Can you imagine how much money would be available to spend on new things? Whole new sectors of industry and entertainment would be able to develop. Your quality of life would be much higher. Also, the trick to this is: don't buy a fridge. Rent cooling. Say you rent cooling for 30 years, electricity bill for the manufacturer. They will provide you with a long lasting machine that is energy efficient! So, who wants to set up a rent shop? You can't do that for all products, I guess, but, for the expensive ones, it's worth it.
In my opinion I don't think light bulb manufacturers reduce the lifespan of their bulbs on purpose; however, this would be a clever practice to increase the number of bulbs purchased by the public. If this is happening and it is intentional and not due to more cost-effective materials that don't last as long, then manufacturers should be reprimanded.