The goal of keeping people alive in an enclosed, self-contained environment whisking through space may be beyond human capabilities for many centuries.I believe this is a very true statement although predicting anything centuries out is a bit dicey.
But note this really has nothing to do with putting a colony on mars. We can certainly get people to mars orbit. SpaceX is even confident enough to put numbers to landing. $150m to $195m to put a lander in mars orbit. This lander will put 2500kg on the surface including four passengers.
There is no question we are up to the challenge of getting people to the martian surface. Can they live there?
A self contained environment works for perhaps a long time but will ultimately fail. Recycling is not going to be 100% so you need to provide for the losses. More important is that it is not growth. People don't want to live forever in tuna cans. You've got to go into the harsh environment to get resources. The biggest resource being more living space. Your sealed environment is going to be contaminated. So it's not really a sealed environment you need but an environment you can clean.
Which is funny since people now claim we can't do it because of dust. They talk of it as if they've forgotten that going around the earth at some places and times can kill the unprepared just as well.
Air filtration is a known technology. We can handle it. We just need to prepare for it. It is not a show stopper except in the minds of those that aren't going to be a part of this next age of humankind. Well a few will later claim they knew it all along.
Update from CJ: [To build tractors on mars] they will need to have either 3-printing, or it's going to be a very long time until they can do this.
Not you too CJ? Why does 3D printing cause us to forget how we do things without 3D Printing? To make a tractor you need the parts and the labor of two people for about 8 hrs. (in the example I'm using.) Enough parts to build two tractors can fit on one lander (2500kg.)
To build the parts you need material and a machinist. Of the 1000 or so parts required, hundreds of them can be made in seconds (3/4" washer x 238) by a skilled machinist. All of them could be produced in a month or so. The raw materials are everywhere in martian dirt (15% iron which is easily made into steel using the carbon in the martian atmosphere.)
It's not just the initial tractor parts we should send. Along with the machinists personal tools we should send parts for the tools like metal extruders. They can make the dies themselves. This quickly gives them a leg up on industrial capabilities. Otherwise they'd have to start as blacksmiths but that works as well until they move beyond that.
That's not to say that 3D printing isn't useful, but it is not required; they will have it anyway.
Something else I'd like to add... some things are not visible to many people. In any city you will find machine shops because they are essential as a foundation for everything else. Alvin Toffler talked about transitions as if they were clearly delineated. They aren't. TV didn't eliminate radio. Radio didn't eliminate reading. ...and so on. We may live in a disposable society where everything is made on the other side of the world, but that's just circumstance, not the only way things can be. Mars will be different because the circumstances are different, but the underlying reality doesn't change.
Another something... what's the difference between big industry and garage industry? Scale. 3D printing is not the only way to get self replicating. The key ingredient is the design. Designs can be scaled, but you have to have one first. 3D printing is just another tool in the toolbox.