If the company that produces open source fails, the software doesn't have maintainers anymore. Unless someone else picks up working on a possibly hugely complex piece of code, you will have an outdated, possibly full-of-secholes relic in no time. Software isn't static.
So many open source projects have a funding problem. A single company pays the 5-15-50 developers it takes to keep it alive, build releases and so on, and then suddenly goes under or pulls funding.
You can't just simply take over the mainenance of a large project at the snap of your fingers. The projects mentioned in the article are the exception, not the rule. The "community" doesn't typically come up with funding to the tune of several million dollars a year, nor does it have the know-how on all the details.
If you need an example, take a look at the commit graph of the oVirt project [1] after Red Hat decided to sunset it. No forks, very little in the eay of new maintainers. Unless you want to risk it, you probably need to move off oVirt sooner or later. (Bias disclaimer: I worked on the Kubernetes/OpenShift integration for oVirt for ~2 years.)
[1] https://github.com/oVirt/ovirt-engine/graphs/commit-activity