Firewalls used to be the perimeter. In a cloud and hybrid world, identity is. That changes where security effort should go.
The old mental model put a firewall at the edge and trusted everything inside it. That model does not survive contact with cloud services, remote work, and personal devices. The new perimeter is identity.
What that means in practice
If identity is the perimeter, then Entra ID is the control plane for your whole estate. Getting it right, MFA, Conditional Access, least privilege, lifecycle, is the highest-leverage security work there is.
Get identity right and almost everything above it gets easier to secure.
It is also where we most often start, because it raises the baseline quickly and sets up everything that follows.