The replacements Bose sells themselves are much more expensive parts, and we know their quality will require further replacement in case the earphones live that long. I'll go as far as saying they designed this to fade away quickly. That way they get the customer to buy that replacement part too.The only Bose ear cushions I have had to replace are the QC3 cushions. Just my opinion, but I think it is worth noting that they are replaceable. Many headphones have cushions that are not. And, you are right. There are many third party cushions at much lower prices with varying degrees of quality. Bluecell is a decent third party brand.
Also, when you purchase replacement parts from Bose, the shipping is free.
After seeing three pairs of cushions on three customers' qc15 fade equally quick and having the electronics in my qc25 break (also fixed the cushions here) after three years, I'm now using some B&W as replacement. The B&W P7 cushions are replaceable and can be swapped in a minute while costing the same as the Bose spares.