It won't get you fares from 1990, but I wanted to point out that you can get historical average airfares for select U.S. cities from 1995 onwards at https://www.transtats.bts.gov/AIRFARES/.You are right it is impossible to find out airfares.
As an example, the airfare to Orlando was $245 in 2018 but was $300 back in 1995. The quoted prices are not inflation adjusted. If you were just going by that, airfare went down in both real (inflation adjusted) and nominal dollars! Note: I don't think these averages are adjusted for changes in destinations. Also, the listed fare averages don't account for baggage fees.
I find I can neither agree nor disagree with this, just because there's latitude for it to be either true or false. I agree that the majority of American's can't afford a "standard" 4-5 day WDW vacation, even offsite, but that's because 80% of U.S. workers are living paycheck-to-paycheck. And though I'm sure people will say that people can just pull their credit cards out, most families who have credit cards would have to pay off enough to even have the available credit to charge their trip. It's honestly crazy scary to me, but it's true.My position is easy. A WDW vacation is very expensive and always has been. It is not for the majority of Americans and never has been.
OTOH, what @MrMcDuck says is true. One can do Disney on a budget and spend a fraction of what the average WDW vacation costs. I have trailer park relatives who go to WDW for a day or two every few years.
I know the resorts are half the point for myself and most here, but a less well off family that is really trying to save on a Disney trip will bargain hunt and find a deal for around $50/night for an offsite, Kissimmee-based hotel with shuttle transportation to the parks and complimentary continental breakfast. They'll have their stale danish and cereal in the morning, bring some snacks to the park, and do QS and CS for the rest. It's certainly not an ideal way to do it because it wastes a ton of time on transportation, but based on how popular those hotels on 192 are during certain times of the year, many people are choosing that option over not making a Disney trip at all.
Then there's Scrooge McLensman. I've mentioned that we pack our lunches at least once per trip, and we've reduced the frequency of trips to every few years because WDW has become "unaffordable". And I'd consider us to be middle-class - or as cousin Isobel says, "Upper Middle Class!".
BTW, I agree with everyone who dismisses the idea that Disney is raising prices "to reduce crowding". They are raising prices "to increase profits". However, I do agree that they are raising prices more during peak seasons to redirect some people away from peak season and towards off-season.