You can have user limits on the non-enterprise plans (Microsoft does this, for example, with Business Premium locked at 300 users or less), or gate other features behind enterprise: Have MFA across the board, but lock conditional access behind enterprise, lock more advanced audit logs & reporting behind enterprise, lock RBAC behind enterprise, or data residency, custom security policies, API limits, etc.
There are numerous other features that are non-negotiable for enterprises to help funnel them into the enterprise plan, while still being able to service medium companies with SSO.
Maybe in 2015 it was an outlier, but SSO is now a non-negotiable and with many of these businesses on M365 business premium, which includes EntraID P2, SSO is now accessible to a large number of companies where it wasn't before. It's no longer some niche enterprise only functionality, it's a bare minimum for business SaaS.
And if you’re trying to negotiate custom, non standard licensing when you’ve only got 300 employees you will likely be a noisy customer in perpetuity.
No offense, that’s just how I’m betting 99% of folks read your response.