Imagine if Lewis and Clark were trying to sell building a shed on the pacific coast.

If Space is luxury then it's a luxury I don't need.

In my opinion space is a necessity- a necessity that because time has been already wasted by not opening the space frontier sooner, has cost us.

Determining the cost is difficult. But in a similar vein, I would had said that had we not had air travel that would have cost us. Now some people believe that air travel is a bad thing, some think cars are a bad thing, some think coming down from the trees was a bad thing, so if you think coming down from the trees was too costly, and cars and jet planes are too costly, then it pretty useless for me to try to argue that Space is a necessity. So to all you people who are on the "Sheryl Crow's one sheet to wipe your ass" side- you are hopeless as far seeing the necessity of Space.

So, I think air travel has value. Obviously the ability to make and fly planes, had an advantage in terms determining whether America won WWII and therefore determined that we have as many freedoms and democracies on planet earth at this time. So obviously in terms of national security, airplanes were a vital necessity.

That really goes without saying, but I think it's pretty obvious that normal commercial flights are also a necessity. Personally I don't fly that often, so if I only looked at air travel by narrow scope of my personal use of airplanes I could be deluded into thinking it was just some kind of luxury and not a necessity. But I do receive things in the mail and I can send things in mail, and depending the distance and other factors it could go by air. So ordinary mail is less obvious thing which in order to have delivered in days instead of weeks or months is dependent on air. Also many items I buy may never to sent by plane, but in order make that product at various stages it's very likely that air travel was involved. Air travel has shorten distance and time needed, thereby increased the productivity of millions of people, resulting broadly in cheaper and better products. Air travel is safer than traveling by road- therefore air travel has saved many lives. Anyways could go on, but I think it's obvious that air travel is a necessity- though I grant it is not viewed so by people who think leaving the trees was a bad choice.

So in comparison with air travel, I think opening the Space frontier is more of a necessity. In quick broad strokes, you have the national security aspect, then you have an improved on long distance air travel, getting to resources and environments which are unique [such as vacuum/low gravity], and most important an avenue to create and transport energy which could exceed all global energy needs. I could talk endless about this, and I have.
But I would like to understand how anyone see a future which does not involve space- assuming you can imagine humans living for a few more thousand years.