There's no good breakdown of the finances of Universal Resort pre-Comcast (because Comcast earnings reports are far more detailed and separate specific businesses unlike GE or Vivendi, also Universal only received management fee/dividends for Orlando from 2000 to 2010 due to 50-50 ownership):
Ignoring investment costs (which are huge in this industry and higher in the most competitive markets, I'd assume around 10-14% of revenue goes straight to investment at least):
The earliest year reported: 2010 (which includes 6 months of Harry Potter at IoA): $1.6 billion revenue for Hollywood+Orlando, $1 billion costs, $600 million in operating cash flow
My guess for 2009 (completely pre-Potter and during a recession): $1.45 billion revenue for Hollywood+Orlando, $950 million costs, $500 million in operating cash flow
That's a healthy operation (it's not post-Potter Universal, but it's comparable or better than most other park operators). The earnings reports from Comcast definitely support
@Disneyhead's point about increased spending on service:
Orlando+Hollywood costs of operation (excluding investment costs):
2009 (my estimate): $950 million
2010: $1 billion
2011: $1.1 billion
2012: $1.1 billion
2013: $1.2 billion
2014: $1.45 billion
2015: $1.7 billion
2015 is the last year before Japan's results are included.
So spending increased slowly at the parks from 2009 to 2013, then they dramatically increased spending from 2013 to 2015 (increased personnel costs and other costs to support much higher attendance and new attractions).
The thing supporting
@Joe's point is that IF there was no Harry Potter, then it's highly likely that Orlando+Hollywood would have been sold to a private equity group or other operator (Six Flags/Cedar Fair) that would have to justify around $3 billion in debt (to buy the parks/hotel stake) and pay licensing fees for the various licenses/rights. I'd find it hard to believe that the parks could support an additional outlay of $200-300 million on interest/debt payments as well as licensing fees each year; that'd bring the margins on the parks to basically $0 after accounting for investment costs. Could the parks have survived in that universe? Sure, but they'd probably have faced significant cost cutting and spending on attractions would be nowhere near what it is when you look at what actually happened.