"It IS technically possible, but very expensive. The key to success is to use Martian resources to their best advantage and to import as little as possible from Earth.

Ample water, minerals, etc are the keys. Since they are known to exist, the next step is to learn how to exploit them efficiently."

I have a nearly opposite view.
I think a key to success is if Mars imports a lot from Earth.
Or said differently, if Mars is importing a lot from Earth that indicates that Mars is successful.
Or if Mars doesn't import much from Earth- it's an indication of a dismal failure.

I think Mars will probably mostly export to Space rather than to Earth, and Space will export to Earth and import from Mars and Earth. But these details of who trades to who, isn't as important as simply the measure of how much Mars is importing- importing a lot means it's successful, importing a little means it's poor.

So a key to Mars being successful is the degree it can import and export. And if it can import a lot [for or by whatever reason/means] it's doing well.

The issue isn't whether traveling to Mars is expensive, the issue is that getting off Earth is expensive.
LEO is halfway to anywhere- and Mars is a fairly cheap anywhere.
The key is making getting off Earth cheaper. And the only way you can make getting off Earth cheaper is by having more volume/traffic leaving Earth.

Suppose that Mars was a habitable planet and suppose we were it's inhabitants. And Earth was habitable planet but no one was living there- yet. And we as Martians were looking at inhabiting other worlds.
Earth is a water planet and Mars is not. [Martians not accustom vast area of oceans could see such areas as wasteland and unproductive. So Earth is this massive body which has less useful land area than Mars]

Landing on Earth is hard but it's not impossible, but getting off earth is quite a challenge. Compared to getting off Mars, Earth is a nightmare. Marian would probably need to invent the idea of using staged rockets and throwing these stages away. What investor or govt would support such a nutty idea?

Martians would have one advantage over Earthlings- they would realize that getting off Earth is the hard part and they could focus their attention upon resolving this problem. They also would also, obviously, have rocket fuel in Earth orbit.