For installations of a few hundred instances or more, some of the SaaS offerings cost more than the engineering salaries it would take to maintain the OSS tools.
Note: I am one of the maintainers of Diamond, a metrics collection tool written in python. https://github.com/python-diamond/Diamond
The OSS tools costs a fortune in human to maintain them, and another fortune in hardware to run it.
(I picked 600 because that was the approximate number of machines we had at my last job, where we used Graphite maintained by one guy, part time).
You included a LOT of redundancy in your OSS list. Multiple timeseries databases. Multiple collection daemons. Multiple dashboards. Multiple alerting systems (Who in their right mind would use Nagios AND Icinga?). You're effectively arguing about maintaining multiple monitoring stacks, some of which are quited aged.
Let's say statsd + collectd (metrics collection) + graphite (aggregation) + carbon/whisper (graphite storage) + icinga (alerting) + grafana (graphing). That doesn't exactly come easy.
No offense but a single graphite is not a monitoring solution. It's just the tip of the iceberg. Monitoring does take a lot of engineering work and a lot of maintenance. You won't get away operating 600 hosts on the cheap, just think about how much are the hosts themselves.