"With QUIC, however, the identifier for a connection is not the traditional concept of a "socket" (the source/destination port/address protocol combination), but a 64-bit identifier assigned to the connection."
Also, Google benefits from a faster and more secure Internet and its employees have the freedom to pursue that in a wide variety of ways. They aren't all mustache-twirling villains.
But I do think the security and privacy implications should be explored. What could an attacker do with a persistent connection?
But a session cookie doesn't allow to transfer a stream to a new IP address during a long request/response.
To be fair to the proponents of QUIC I add the next sentence, which depicts a use case:
"This means that as you move around, you can continue with a constant stream uninterrupted from YouTube even as your IP address changes, or continue with a video phone call without it being dropped"