you are partially correct. Glass is used for several reasons. It is a very stable material and will hold its shape well for thousands of years. Glass can also be polished to a high degree of accuracy without having defects. The expansion properties are very favorable as well. Glass doesn't corrode and is easier to mold into shape than metal mirrors.

you are partially correct. Glass is used for several reasons. It is a very stable material and will hold its shape well for thousands of years. Glass can also be polished to a high degree of accuracy without having defects. Another major reason is the expansion properties are very favorable as well. Glass doesn't corrode and is easier to mold into shape than metal mirrors. Glass is relatively cheap and readily available.

Some useful references are found here ,here and here.