50 SRE's sounds like they could keep borg running. There really isn't that much details on what borg is made up of outside of a 2015 research paper[1].
Just to address the parts you laid out.
1. Keep Borg running
What would entail running? There are plenty of small teams that manage hundreds of servers and virtual machines through monitoring, deployment, networking, decom, etc lifecycle. This seems counter to purpose of cloud computing, commodity hardware and abstracted compute resources that can be deployed on-demand. Not to mention redundant sites, availability zones, <insert your cloud providers name for High Availability(HA) feature here>. Is this the part that SRE's maintain?
2. Machine maintenance
From documentaries[2] and other comparable data center operations this would seem to be handled by on-site datacenter staff. Disk replacement, physical replacement, network cabling, etc. Without any other operational info I would doubt resources would just be dedicated to borg as its a bit of a overall technician site work. As for OS and software updates and again without any other info or insight would seem to fairly automated after passing testing or at least passed to another team that just handles updates.
3. Machine procurement
Again commodity hardware or if its all custom still that would seem to be more of a EE task for build out and accounting for purchasing. Otherwise at Google's scale you would just get pre-populated racks delivered and replace the entire rack when 51% of the machines have failed. This doesn't really seem SRE or developer specific. It would also be happening for other services if they are already aren't abstracted from the underlying hardware/network layer.
4. Regulatory changes
I'm not sure what this would be in relation to a job processing system? Is this checking where you are saving data? Seems like a feature that is built once.
I agree that if you have follow-the-sun and need to be up with 5 (9's) 50 people in one time zone would be hard to ensure that but once the foundation is setup its just (50) people in a different location or timezone there isn't really anything too different about what they are doing.
If you have additional information or insight that would interesting to hear about.
[1] https://research.google/pubs/pub43438/
[2] https://www.youtube.com/watch?v=XZmGGAbHqa0