Again, my reading of the sales agreement (contract) is that Membership Extras can change or can be eliminated completely, at any time, and without notice, and there is clear language defining Disney's right to do so. However, the contract also clearly defines the requirements for eligibility for those Membership Benefits, and that language cannot be changed unilaterally by Disney. Think of it like a home mortgage with a variable interest rate. The lender has the ability to raise or lower the interest rate based on certain criteria, but they can't unilaterally decide 5 years down the line to add a balloon payment requirement or modify the term (duration) to the note.
In other words, Disney is bound by, and the owner enjoys, whatever eligibility requirements for MB's as defined in their specific contract, but Disney can removed those MB's across the board at their discretion.