Universal has had tremendous success with Potter- they spent a lot of money and got a lot of money in return.
Disney, meanwhile, didn't spend millions of dollars on building new attractions and offerings... and still made a ton of money off of returning guests with nostalgia.
All theme parks, even WDW, survive on frequent updates to their offerings. Nostalgia carries you only so far.
The problem with today's corporate Disney is that failure to invest properly in WDW has hurt company profitability.
Back in WDW's glory days, Disney used to invest ~25% of Parks & Resorts (P&R) revenue back into the product. As a result of that investment, margins typically were in the 18%-24% range. Disney invested a lot of money and made a lot of money in return.
Last year, Disney's P&R investments were down to 15% while margins were under 16%, lower than at any point during WDW's first 30 years of operation.
P&R used to be money-in-the-bank for Disney. Now, P&R's margins are actually lower than the rest of the company. WDW is dragging the company down.
Meanwhile, last year Universal invested 26% of its Theme Parks segment revenue back into its parks. Their margin was over 31%. Universal Orlando pulled Comcast's earnings up.
The old adage "It takes money to make money" is true.
Disney needs to invest intelligently in WDW in order to see the strong returns it once did, in order to see the strong returns Universal is experiencing today.
Instead, Disney spent more than $7B over the last two years buying back company stock. Besides helping boost the stock price so executives can collect bigger bonuses & exercise their stock options, buying stock that's already trading at a PE ratio of 21 isn't a particularly good idea.
Disney has money available to invest in WDW right now. They simply choose not to because it's not in the best interest of their senior executive management.
Things will change. Disney will invest in WDW once again, sooner rather than later. But the low rate of investment over the last 10 years or so really hurt the profitability of WDW.