>Surely the number of eyes looking at that codebase will vastly diminish though?

I don't think the "many eyes" theory ever panned out. There are projects used by billions, and on which thousands of other FOSS projects work, and only have like a handful of people working at the codebase (and less catching bugs)

>Also Google have golang, which I'd imagine would be their target now, not python 3 (for migrations)

Not really, Golang was never Google's official/mandated language. Just a language some Googlers made with Google's use cases in mind. Google still uses C++, Java, and Python (plus some Dart). Some things use Golang, but not in some top-down "Googlers shall use Golang" way.

Internally, all around. A 2018 answer by a Google engineer:

Build system automation of all kinds (although the main build tool is in Java)

Test automation

Deployment automation

Many services configure their job placement and options with a Python-derived language (not mine)

Monitoring configuration and dashboards

Data analysis using Scipy, Scikit, Numpy and various kinds of notebook UIs

Configuring Tensorflow and analysing the results of ML projects

Hundreds of little command-line utilities for all sorts of small code and data manipulations

Thousands of internal websites

The youtube.com site was still Python last time I looked

One thing I know of off the top of my head (because I had to deal with it last week) is the gcloud suite of tools for GCP, which will only run with 2.7. Thankfully the various GCP libraries for interacting with the platform do work on Python 3.