Plus there are a couple of other factors: inflation and new airlines.
Few things are truly the same price as they were a decade (or more) ago, and this is simply due to the standard rate of inflation. Every organization (company, government, non-profit, etc) takes the Consumer Price Index into account when forecasting and budgeting for subsequent years, and this index has been roughly around 3% every year for the past couple of decades (fluctuated a bit since 2008). Most companies bake this into their pricing strategies, thus it causes the price to increase each year.
This is ok, though, since, in general, many wages are also adjusted to account for the higher cost of living (COLA). That raise we get usually isn't a raise; rather, simply an adjustment to make our wages equal to cost of products/services on the market. As such, prices may increase, but many times, the ratio of price/income is (hopefully) relatively normalized.
The second (and I maintain the most prominent) factor that caused the precipitous drop in airfares during the last 10-20 years was the introduction of new players in the airline market. When a competitor enters, their primary goal is to generate market share instead of profits, especially in the first five years. Once the business has a loyal following, profits will come.
Southwest is credited as the main driver of lowering legacy airline prices, as they had a business model with no hubs, operating out of smaller regional airports and they pre-purchased fuel. This enabled them to significantly undercut their competitors' pricing and generated a lot of market share. A fact that SWA is now capitalizing on, despite their pricing now equaling or exceeding the legacy airline options for many markets.
In addition to Southwest, the airline industry saw the introduction of AirTran, Frontier, Midwest, Spirit, Direct, and Alliegence, just to name a few of the more recognizable companies. With the market flooded by "low-cost" competition, the cost of airfare dropped significantly.
However, there was an unintended consequence, as quality also dropped. To offer low prices and remain viable, the "low cost" airlines also removed amenities and customer service took a hit. The airline industry shifted from a well liked industry with a high reputation for service (and maybe a bit of adventure) to one of mass transit. Instead of being a high priced service most people would not be able to afford to use frequently (much like our beloved Disney, BTW), it became and industry focused on moving the most number of people for cut rates. In turn, many Americans began to feel they were entitled to fly and the airlines should keep prices low to ensure even the lower income among us were able to use the service at their leisure. Thus, the thread's title.
Personally, I remember the days when $300-$500 airline ticket was the norm, at the inflation of 20-40 years ago, and most people drove to their destination as flying was that special thing people did rarely (and on special occasion). I just hope that when prices increase and the industry normalizes back to the way it was, which it seems to be doing now, that the amenities will return to make flying special again. The competition of the "low cost" airlines did a lot of damage to the quality of the product, in my opinion.
In summary, what happened was that airfares are returning to their norms, as the uber-low cost model seen in the past decade was never sustainable; rather, it was a measure to gain market share. With fewer new competitors and the successful "upstarts" achieving legacy status, there isn't as much incentive for the airline industry to slash pricing below market value and scrape by in lieu of making a profit.
I check every day, but have not seen it go below $400.....Good Luck!
. Our flights are via JetBlue. Never thought about checking United.....are your flights direct? Thought JetBlue is our airline of choice if at all possible (we love the spacious seating, free checked baggage, free snacks, free seat selection), however if prices keep increasing like they have been lately, we may have to consider "the competition".
and that is 
with two kids!
