Diffie Hellman doesn’t ever send the key over the wire, that’s the point. There is nothing to decrypt in the packets that tells you the key both sides derived.
Unless they break ECDHE, it doesn’t matter if RSA gets popped.
Diffie Hellman to the best of my understanding also relies on the same hard problems that make the public key cryptography possible. If you trivialize factoring of big numbers, you break both RSA and the original DHE. Not sure how it will work for elliptic curves, but my instinct tells me that if you make the fundamental ECC problem easy, the exchange will also go down.
Unless they break ECDHE, it doesn’t matter if RSA gets popped.