Kubernetes has a lot going for it, but it isn't (yet) anywhere close to Borg in terms of scalability. 5000 maximum hosts in a cluster is a drop in the bucket for a Borg cluster. Google was publicly talking about 10,000 machines as the median size almost 5 years ago.
As for Prometheus, I like the rules engine VERY much (unsurprising, since they basically reimplemented borgmon, there). I'd take Viceroy any day over Grafana, though.
I have deep experience with operating Kubernetes and no experience with Google's Borg but I imagine Borg is significantly more challenging to operate? Kubernetes is designed to be a workload scheduler that a three or four person team can operate.
I imagine if you're Borg SRE, you'd say "yes, it's significantly more challenging to operate", but for the tens of thousands of other engineers? Not really, although there was a LOT of copypasta in terms of building new configs for services.
As for Prometheus, I like the rules engine VERY much (unsurprising, since they basically reimplemented borgmon, there). I'd take Viceroy any day over Grafana, though.