In general, paying GH tends to be much cheaper than employing a team of SREs that'll keep your on-prem Git instance up and running 24/7 (esp. including things like backups, disaster recovery, and similar good practices necessary for any enterprise-ready SaaS).

Something tells me that if you can schedule upgrades "outside of business hours", your scale is small (because in a global company, it's always "business hours" somewhere), which makes the cost argument even more prominent.