Sigma Lens

I realised that my Sigma lens 70 - 300mm DL Super Macro had some compatibility problems. When the shutter was depressed, I heard the mirror click, but it would not come down. It could also be faulty aperture blades. An error message was recorded in the LCD, stating that there was connection problem with the lens. I bought it to Sigma dealer for a check, they replied that the lens was an old model (7 years old) and there was no spare part for the lens to be made compatible with my EOS 450D. I tried to explain that my Canon EF Lens that was as old as the Sigma lens had no problem with the digital camera. The dealer explained that there was nothing she could do but could give a discount if I buy a new lens from Sigma that could be compatible with the digital camera. I was disappointed by the reply obviously and was convinced that I would not buy Sigma lens again.