Here in 10 years maybe, the coupling between Google and it's infrastructure is like the head over body. Cutting cloud out surely delights TK, but before that the other SVPs will make sure TK got to rebuild it's engineering workforce and leave the exiting on themselves. In other words, Google is totally fine to retain the core of GCP.
I think the bigger issue would be taking the tech developed by Google and porting it over to GCP as a separate company (i.e. like they did kubernetes, bigquery, etc.)
Alphabet Cloud and Google could work out some easy service agreement. Worked for Coca Cola and Coca Cola Bottling company.
You cannot build a Borg over kuernets, like you cannot build baremetal VM over VM. One can get the abstraction. But the cost would be totally messed up, and some of the performance characteristics would be so bad that the whole ads business ecosystem probably would collapse.
GCP can build on top of Google tech, but not the reverse.
I might be wrong though