Mirror or light tunnel collapsing

A weakness that you might experience with your projector is the collapsing of the light or mirror tunnel. Not sure if this is common to DLP projectors only. The symptom is that you see grey bands or shadows on the edge of the TV image - this is caused by a mirror getting in the way of the bulb light. The glue holding the mirrors together seems to weaken over time (3000 lamp hours in my case) and causes one or more of the mirrors to loose its position. For solution, search for "mirror tunnel" or "light tunnel" under all projectors. The problem seems to be quite widespread. I managed to fix my LG's.

I wonder why the manufacturers don't use a rectangular glass tube that is mirror plated on the long sides ?!?! Suppose they want to sell new projectors after lamp or mirror tunnel expiry!