Of course if you had provable decentralized timestamps you wouldn't need mining to begin with. Just check the timestamp of transactions and the first one is valid and the second is invalid.
I don't believe in "provable decentralized timestamps" and I did not expected to have these. I was thinking about some way how all nodes in the network could agree on "what time is it" with reasonable accuracy. The "reasonable accuracy" would be good enough to make near-real-time difficulty adjustments but it would be not sufficient for transaction ordering so you would still need mining. The near-real-time difficulty adjustment would remove the need for "emergency updates" killing the difficulty if 99% of the hash power would suddenly disappear into thin air.