Although a broken mask strap is not common, a spare mask strap is a great item to have "just in case".
All of our masks are made using silicone, and all of our mask straps are silicone too. Silicone is not prone to dry-rot nor will it deteriorate over time. However, a broken strap may occasionally happen. We stock all of the replacement straps for the masks we carry, and our straps will fit just about any other brand of mask too.