> More development resources?
Not directly. I expect the gains to be around the legal and organisational stuff you need as a project grows (e.g. trademark registration, formal governance), as well as publicity.
> I suppose it at least to some extent allays whatever fears there might have been that it will go "open core" in some ways, as e.g. InfluxDB recently..
That was never really a risk, as Prometheus has never been a commercial effort controlled by any one company.
> That being said, what is the killer feature here?
Prometheus is perfect for monitoring any sort of operational timeseries, and is designed to do so in a way that scales both technically and organisationally.
For example collectd is an on-host daemon that you have one of per machine. That may work when you're small, but with many services that will become a bottleneck as each need to be added and stragglers may become an issue (Twitter seem to have hit this, see "Lessons Learned" https://blog.twitter.com/2016/observability-at-twitter-techn...). The Prometheus approach of one exporter per target scales far better.
For another example if two different teams (say infra and database) have two different views of the world, with a collectd approach then the infra team likely wins. With Prometheus monitoring can be decentralised so infra can view a machine as mach1{rack="ch",datacenter="ma"} and database as mach1{owner="app2",env="canary"} each in their own Prometheus server. This gives teams more control by decentralising monitoring.