If dues rise in lockstep with Inflation then what costs $5 today WILL cost $25 at some point in the future. But it will still only be worth $5 and that is what $25 will be worth then. I remember buying gasoline for 22 cents a gallon. Hamburger for 25 cents a point, etc. etc. So, now I pay more, but I also get paid more, so there really isn't a problem. Inflation is deliberately used as a policy (by the government and the Federal Bank), to give people the impression that their wages and their property are going up in value, when really they might be remaining the same.
The trouble comes in if Disney goes on a streak where they significantly increase Membership Fees above the inflation rate. Then the MFs actually WILL be costing more. During the meantime, Disney definitely is, and has been, increasing the cost of their resorts and hotel rooms significantly faster than inflation. To the point where it is starting to be priced out of the range of middle class families. So, the 'hotel renter' pool is getting smaller and smaller, proportionately, but there still is enough demand that Disney usually fills those rooms. If it costs more to stay in a hotel and if the MFs have gone up by the same amount, then it will all still balance out in the end. Unless the renter pool shrinks too much, and if prices continue to go up at this level, Disney will reach that point.