The glue hardens and can crack with age, happens with every kind of glue known to man. The fabric that the glue is adhered to to hold the glass to the top deteriorates.
Sure I do not want that to happen to my car, nor do I want to be following someone that it happens to, or any part falling off the car in front of me for that matter. BUT the fact remains that it can happen. Will the rear window come flying out and kill someone? Not hardly. It is larger than the opening and is retained by the defroster wires if/when it does happen. Frequent inspection of the seam can prevent complete failure before it happens. I check my car over fully at least every other week, rear window seam included. If/when the seam looks to be separating, guess what, I will have to top replaced. Goes with the territory.