The magnetic shield has nothing to do with trying to keep people on the surface.

The Earth's magnetic Van Allen belts deflect most of the sun's nasty radiation streaming at the planet. These belts are created by the molten iron core of the planet. Mars no longer has this magnetic shield, and the sun's radiation burned off the atmosphere. If we generate new atmosphere, it will just get burned off again unless we can generate some way of deflecting that radiation.

Aren’t most terraforming estimation put the time frame of atmosphere erosion at a few 100M years?

If we you can create an atmosphere it should last for a long enough period to be worth it, heck it might outlast humanity.

That still doesn't solve the radiation problem(at least on mars). What's the point of colonizing Mars if we'd all have to live underground anyway? Might as well try doing that on Earth(or the Moon) first.

I think it's for solar and cosmic radiation.