We already know how to build colonies, even if we haven't done it on another planet. Turning a subway or megamall design into a Martian colony is an engineering problem. Terraforming is a fundamental science problem. We're still a long way from cracking it.

It's more than just an engineering problem. It's also an economics problem. What is going to pay for that settlement? Are there enough mineral resources on Mars to justify the cost? Or will it be paid at taxpayer expense, for no significant benefit over unmanned missions or brief manned missions to Mars?

So yes, it is easier to build a settlement than to terraform, but I don't think you could justify the cost to build a self-sufficient settlement. Heck, if we start running out of land to settle on on Earth, there's always the sea to settle, which requires many of the same engineering considerations to settle but at much lower cost to get us there and do trade with the rest of the planet. I suspect we'll be seeing either underwater or raft-based settlements long before we see settlements on Mars.

A lot of people would like to live on Mars, given the chance. Meanwhile, people are growing richer and technology is advancing. It's fairly likely that at some point there will be a point where people can pay their own way to creating a Martian colony.

A lot of people might like the idea of living on Mars when there is absolutely no risk of that actually happening. But really there is not much point in living on Mars for extended periods of time, there's very little that humans could do that machines couldn't do as well and a lot cheaper. Combine that pointlessness with what would in practice be an an isolated, risky and uncomfortable life, and I suspect that the pool shrinks down dramatically.

Terraforming is not necessary for settlement. It isn't even obvious to me that Martians will choose to do it down the line.

(I feel like I should be using terms like "minimum viable settlement" around here!)

Terraforming Venus you'd need to figure out how to make a planet rotate 1000km/hr faster. Terraforming Mars you'd need to add mass equal to another planet at least the same size.

Domes will do fine then.

But as jsnell said below this: A lot of people might like the idea of living on Mars when there is absolutely no risk of that actually happening.

While I think that we're still a good ways from pushing this planet to it's limit (at assuming renewable energies come online quickly enough to mitigate population growth), eventually we'll want more room. And living in contained units (underwater, in space, or on other planets, ) doesn't seem like a worthwhile solution. So we're either talking population control or making more livable space. Surely, a thousand years into the future, we don't want to only have one livable planet to our race. And (maybe I'm wrong but,) terraforming what we have seems more likely that traveling to other Earth-like planets.

I'm not crazy enough to think that this is something we can overcome easily, or during my lifetime, but I think it's more of a "if we don't get started now, when?" type issue.