Deploying zero trust software-defined perimeter (SDP) architecture is not about completely replacing virtual private network (VPN) technologies and firewalls. By and large, the firewall demarcation points that mark the inside and outside are not going away anytime soon. The VPN concentrator will also have its position for the foreseeable future.
A rip and replace is a very aggressive deployment approach regardless of the age of technology. And while SDP is new, it should be approached with care when choosing the correct vendor. An SDP adoption should be a slow migration process as opposed to the once off rip and replace.
As I wrote about on Network Insight [Disclaimer: the author is employoed by Network Insight], while SDP is a disruptive technology, after discussing with numerous SDP vendors, I have discovered that the current SDP landscape tends to be based on specific use cases and projects, as opposed to a technology that has to be implemented globally. To start with, you should be able to implement SDP for only certain user segments.