Companies of that size are common. It would in isolation even be profitable to serve them. The problem is if you introduce a middle tier that includes SSO, many enterprises will go for that instead of the expensive enterprise tier you want them to buy. Basically, you sacrifice medium companies as customers in order to chase after that sweet enterprise money.
Companies of that size are served by the "enterprise call a salesperson" offering. If you really don't need all of the other features you can probably negotiate a discount.
That makes sense, but I still think there are other features that can be gated behind enterprise to help make sure that doesn't happen while still providing SSO for smaller companies.
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.
