An Earth-like atmosphere would need to mass 27 tons per square meter on Mars vs 10 tons on Earth, due to the lower Martian gravity. That would be 3900 trillion tons total. Current loss rate is 95,000 tons a year. If the loss rate increased a thousand times to about 100 million tons/year. that still gives a half life of 20 million years, which is long by human standards.

There are several ways to reduce the losses. One is to put a magnetic shield "upwind" of the solar wind, and deflect it off the planet. That's effectively what Earth's magnetic field does.

Another is to dome the planet. Surface pressure depends on the weight of what is above the surface. It doesn't matter what that weight is made of. 27 tons is a lot per square meter. It would be more than 10 meters of glass thickness. So you can build a greenhouse the size of a planet and keep the atmosphere from leaking out.

Just because the top of an atmosphere being exposed to space is natural doesn't mean it is required. You can have several km of air below the dome to get an outdoor feeling, and leave the taller mountains sticking out into space if you want.