It's partly a grim sort of math where the public would be forced to support continued funding for the Mars affair. The Moon landings generated some interest for about a year, but nothing substantial over the long term. Having a number of hostages stranded on Mars and completely dependent on public funding will ensure Mars spending in perpetuity.

Now and perhaps for the next few decades, Earth's most hostile environments would still be more habitable than the best Outer Space has to offer. Early colonists to new lands on Earth would already be self-sufficient the moment they arrived, whereas Mars colonists would be dependent on Earth for decades to come.

It's not even certain that this mission would add much in terms of know-how or capital investment into economically sustainable (instead of subsidized) space colonization. 20 years of space shuttle and a decade of ISS didn't make Earth Orbit more inviting to humans. The costs to sending humans in space has not changed in decades and commercial exploitation of space has not been helped by government boondoggles.

I think it's rather foolish to pursue planetary colonization before at least Earth's orbit has become economically accessible and habitable.