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A mirror certainly doesn't need to be made of glass. What is required for the mirror are very specific physical properties. Thermal stability is the first. For photo this is paramount. Dimensional stability when the mirror is tiIted - trickier with larger apertures. Lightweight - don't underestimate the ballast that the weight of glass gives to the engineering of a scope. This last point is purpose governed.
Resins can be an interesting alternative. The weight advantage is significant, particularly for eq mounted instruments. Their greatest problem is dimensional stability from thermal variations. Producing stress free large aperture resin blanks is not easy either. I'm sure that there is R&D going on into these. At the moment I can see cost as the main limiting factor. Glass has so far proved to be the most versatile and predictable at the moment. Borosilicate glass types in particular.
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