They both work perfectly fine for the very limited use cases they were meant to, at the time they were made, with the limited resources that were available.

Just like foot is the best way to travel and commute to work every day, as proven by the last 10000 years of people using theirs.

No, the key difference is that Nagios has perfectly viable alternatives (Zabbix admin here and it's not the only one). Jenkins however has no alternatives.

Bamboo or whatever it is called now is typical Atlassian crapware. Expensive as fuck, eats more resources than the stuff it builds, and did I mention it is yet another half assed product that got shoddily integrated into the usual Atlassian lineup?

Gitlab CI is great for anything that is code (think build, test, deploy), but it is not suited for abstracting "non-development" jobs which can perfectly be automated in Jenkins (e.g. creation of a dev environment with fresh data from production). Plus it is Docker and the runners are polling - which means at minimum 10s startup time compared to milliseconds for a Jenkins shellscript job running on a ssh connected slave!

Github and friends are cloud which is a big no-no. We're placing too much power in the hands of AWS, GCE and Azure already, no way in hell it is a good idea to put private source code to a cloud provider.