In an Enterprise, you almost never buy 'a database'. There's a solution/application that you buy, that will have a supported DB backed (usually Oracle or MSSQL). That solution is mostly picked on business capabilities and functionality. The technical component is only one aspect of the requirements, which cover business capabilities/features, technical fit within existing environment, data migration/compatibility, vendor support, vendor credentials, the amount of training required, the amount of business change needed, etc.

These projects often cost $10m+, once you factor in the technical implementation and integration, business process changes, training for business staff, training for IT staff, transition costs, etc.

So if you want to argue that the business should get the solution that fits their requirements the second best, because the 'best' one has an Oracle backend, you had better come up with a shit-hot reason that can't be solved with 'hire a couple of Oracle engineers'.