Haha, 50,000ft? Now you're just breaking my balls.
However, for everyone else confused about the planes not diverting, whether the planes can fly to Florida or the Equator is totally immaterial to the film's plot. You've missed key points of dialogue that explain everything:
Here is the character Trudeau (Fred Thompson) addressing the tower after the runway lights go out and the control tower systems go down, but prior to the tower knowing that terrorists now control the airport :
Attention all controllers. Attention. We have a code-red alert. (PA)
Every aircraft approaching our sector..and not already in our landing pattern.gets diverted to their alternate airport. Everyone already on approach or inside our pattern.. ..holds at the outer markers
.
Stack 'em, pack 'em, and rack 'em.
So, the planes closest to the airport are held at the outer marker in a landing pattern.
Then, after revealing himself to the tower and telling them he's taken over the airport, Colonel Stuart says to Trudeau:
You have two more minutes to
advise inbound aircraft..to hold at their outer radio marker
. After that, you will be able to receive only.
^^ Stuart, now having control of the tower, uses the planes that are holding at the outer maker as ransom for Esperanza. They can't go anywhere. The terrorists have control of them.
Then, Trudeau communicates with the planes in the holding pattern, and because he can't tell them the truth or Stuart will crash a plane, he merely tells them to hold as that's what he's been instructed to do:
This is Dulles approach to all aircraft..holding at "Potomac" vortex. We are experiencing ome technical problems. Weather conditions, which have deteriorated.have now been complicated by an unforeseen human factor. This has affected all of our electronic equipmentand their redundant backups. As a result, our nav and approach systems are down. We expect to lose voice in another minute.
We want you to continueholding at the outer marker as directed
and wait for further instructions. As soon as we're back on line,we'll expedite your landings on a fuel-emergency basis.
^^ Under normal circumstances, those planes would divert. However, those planes can't go to other airports because the terrorists control them
and the planes don't know that
. The planes think it's a normal weather/instrument delay, and are told to stay put - so they follow those instructions as they are told everything will be okay once they sort it out.
When the tower finally gets through to the planes using the beacon to explain about the terrorist situation, it's too late for them to fly to their alternate landing destination - they don't have enough fuel. All airports nearby are closed and they don't have enough fuel to fly to Florida or the Tropics.

listen buddy,
apparently you know nothing about flying or even how gravity and physics work.
If you're 50,000 ft in the sky, you turn the engines off and let the plane gradually fly dropping altitude very slowly. then when you get like at 10,000 ft, then you turn the engines back on and gain some altitude so you can repeat the process.
From DC, you could reach Barbados, Miami or Tampa in no time. Also you have to consider that since the Earth is round and you're flying south, gravity helps you too.
That's why flying to countries below the Equator, like Argentina or the South Pole is quicker, because gravity pulls the plane south, whereas flying from south to north, consumes more fuel. Why do you think airline tickets to say from NY or DC, to Vegas are cheaper than say South Africa to JFK.

The terrorists' plan was just to take over the airport until Esperanza landed. That would've worked with or without the weather conditions. The blizzard merely bought them more time, and endangered the circling planes even further. Had there been no snowstorm, Stuart and co would've just hastened their escape.
They tried to bring that point up in the 'Everything wrong with' youtube video but again they were overanalyzing a simple plot that anyone could follow.
Also, the planes don't know that terrorists have taken over the airport, so that's why they elect to stay in a holding pattern instead of heading for another airport straight away. They just think it's a normal delay. They would never want to fly 2 plus hours to another airport on Christmas Eve if there was no emergency. They want to stay there for as long as possible because Stuart and co have assured them it's ok, it will be cleared up soon. Until they realize something is wrong, it's too late to divert to another airport.
My name is Hendry William French

They say in the film that there's a snowstorm enveloping the entire North East and it's closed pretty much every airport in that region. Also, the character Trudeau says National is shut down and "iced".
Dulles has stayed open I would imagine because either a) the weather is a little less severe where they are or b) being a major international airport handling air traffic worldwide, it has better equipment and facilities for coping with heavy snowstorms that help it stay open over the other, smaller airports.
I haven't seen this in a long time but your post makes me wonder- how could the terrorists know in advance that the airports in the North East would be shut down and "iced"?
There is no way an operation that took a lot of time to prepare could anticipate climate that severe.
So maybe the OP has a point.

What about Baltimore, National, Philadelphia, Richmond, Newport News-Williamsburg, Norfork, just to name a few?
They say in the film that there's a snowstorm enveloping the entire North East and it's closed pretty much every airport in that region. Also, the character Trudeau says National is shut down and "iced".
Dulles has stayed open I would imagine because either a) the weather is a little less severe where they are or b) being a major international airport handling air traffic worldwide, it has better equipment and facilities for coping with heavy snowstorms that help it stay open over the other, smaller airports.

What about Baltimore, National, Philadelphia, Richmond, Newport News-Williamsburg, Norfork, just to name a few? Within 200 miles of DC there are over dozen commercial airports capably of handling large jets. No need to fly two hours away, and that does not address the fact that shutting off Dulles ATC would leave the planes blind. They would simply tune to a different airport or regional control center.

There were planes that did divert - they tell you in the movie that the flights with enough fuel were sent to Atlanta, Memphis and Nashville. The planes with less fuel (Holly's plane) stayed in a holding pattern above DC.
Miami is a three hour flight from Dulles, and Orlando & Tampa are 2.5 hours and Bahamas is four hours. The planes in this movie had 2 hours extra fuel.

This was by far the stupidest part of the movie. Who ever wrote this film knew ZERO about how commercial aviation works. Planes will not simply circle their primary destination until they fall out of the sky. They have backup airports all along their route. Plus there are a dozen commercial airports within radio distance of the DC metro area. They would simply tune to a different tower or regional control center and be on their way.