No, the analogy doesn't hold.
The analogy would work if Disney were only changing from flexible dates to specific dates. In that case, you're right. The flexible date upcharge would not come with a decrease in price of the base ticket. On average, that's true. Average ticket price will be higher, plus the upcharge to make your tickets flexible.
However Disney is making a second change. Not only do you have to pick your dates, the dates also have date-specific pricing. Disney is specifically advertising VALUE dates.
I'm suggesting something like this:
Current: 5 day park hopper, $460, flexible dates
Future: 5 day park hopper $500 average, no flexibility. $600 for flexible dates
All I'm saying is that the $500 average in the new structure might consist of some dates that are $400, others that are $600, and everything in between. People visiting during the $400 time would see a price decrease from the current $460.