Providing that your speakers don't have foam surrounds they should last a very long time, unless you expose them to very high levels of UV. Bright sunlight will destroy many different materials over time. The old Magneplaners had a problem with it.(they use a UV resistant adhesive now). I am sure you have also seen an old loudspeaker with crumbeling foam around the woofer. The Rodgers Ls35a's I bought in 1978 still sound as good as they did when new. I still use them in my bedroom. 8 years old is just broken in!
speaker degradation question
I know this isn't a brilliant question, but every salesman I speak to obviously tells me to replace my Linn Helix front channels because they are 8 years old. I burned them in properly and they still perform perfectly. I am told to upgrade because they were not designed for both HT and Stereo. Thoughts and suggestions appreciated, thanks.