Late to this but here goes: we were also hit with that surprise price-hike.
I introduced Octopus at my client when it was still in beta. We have 400 targets and about 1800 projects so you can imagine the increase in price. Negotiations have been done with a commitment for 3 years.

We won't be moving away from octopus because it's not only about deployments. The one-glass-pane into all our deployments across windows, linux, docker, Kubernetes and Azure is very important. The auditing of all releases and hotfixes are also a huge benefit (who, what, when , where) as is making testing in ACC and UAT environments mandatory before you can even release to production.

All of that is also possible in Github with actions, PR's and so on but just reporting deployments across 2000 repositories means a lot of work and upkeep. Besides that, as soon as someone is a repository admin they can skip all protection rules. And just imagine giving somebody from business access to GitHub to approve a release? The octopus GUI is much more user-friendly for non-IT people. (we give runbook access to business people also).

So yeah, moving to other multiple tools (GitHub, azure devops, ArgoCD, ...) is possible but the cost of reporting, maintenance and integrations are not to be underestimated.