A better measurement would be how many of those projects are still relevant in 10 years from now.
Also consider that a project needs to not only have existed, but to already have an established track record of being vendor-neutral before it can join.
Kubernetes joined CNCF on March 10, 2016. Prometheus joined May 9, 2016.
In 2017, these 10 projects joined:
- Linkerd
- gRPC
- CoreDNS
- Containerd
- rkt
- CNI
- Envoy
- Jaeger
- Notary
- TUF
Of those 12, only one (rkt) has been archived. Do you honestly believe the majority of these will be unsupported 10 years from now, let alone 10 years from when they were added (or started in the first place)?