Well I have plenty of speakers with poly cones that seem to be getting along just fine.
That being said, I did run into a problem with a set of midrange drivers ( Elac 4/200 from IMF Supercompact II) that I discovered when doing a refoam. At first it appeared that the edge of the poly cone was brittle, but as I inspected further I found the poly cone was cracked near the voice coil. Seems the entire poly cone got brittle over time.
I see some have suggested yours having been overdriven or hundreds of watts poured into them. Maybe , maybe not, but certainly my midrange drivers did not have the problem of high wattages hitting them or massive amounts of excursion causing the cracking.