The argument of technical impossibility seems implausible, although it would no-doubt be difficult. Whether there can be enough engineering will mustered within the company to separate out the AdTech stack from the rest of Google's services is another question.
In terms of how the AdTech stack could look afterwards, here is a paper (I authored) advocating for the introduction of an interop layer such that users could choose which advertising network to use with Google's products and services: https://doi.org/10.36633/ulr.1113
In terms of how the AdTech stack could look afterwards, here is a paper (I authored) advocating for the introduction of an interop layer such that users could choose which advertising network to use with Google's products and services: https://doi.org/10.36633/ulr.1113