[FiatLuxDave]

While there are indeed big challenges to making the Martian atmosphere thick like the Earth's, I wouldn't buy the statement "since Mars has 38% of Earth’s gravity, it can only retain an atmosphere of about 0.38 bar".

Compare: "since Venus has 90% of Earth’s gravity, it can only retain an atmosphere of about 0.90 bar".

The relevant thing is the gas escape rate. Aside from the magnetic field, this is dependent upon the temperature, atmospheric composition and the escape velocity. Without plate tectonics to replenish it, even Earth would eventually lose much of its atmosphere. Note that Venus has a weak magnetic field also, and the interaction with the solar wind is believed to be one reason why there is so little water in the Venusian atmosphere.

So, if you were to add enough gas to the Martian atmosphere, it would have a higher pressure until the gas was lost over thousands or millions of years.

I wouldn't try aiming for terraforming Mars with today's technology, but that doesn't mean that tomorrow's technology might not be able to. Personally, I think Venus is the better choice for terraforming anyways.