My opinion about Mars is evolving, it would seem.
I used to think it was useless for us to go to Mars, and it still is. However, what’s changing is that we are going to go to Mars anyway, and that’s really kind of a revelation that we will eventually become a multi-planet species.
The problem with Mars, I think, is it’s too small.
The size of the planet means two things: one, the planet’s central core cooled off early on it its history, essentially destroying the magnetic field of the planet, and two, without enough gravity you’d need at atmosphere much deeper than Earth to achieve the same pressure.
There’s a third problem, too. No nitrogen to speak of.
These problems would not be solved by dragging the planet closer to the sun. It is basically, as far as we can tell, a dead planet.