I agree that developing Mars is going to be a centuries-long process...
Its not going to be done in the next 100 years, given that we don't seem to have any prospects for at least the next 20 or 40 years for a vastly more efficient way to move people and materials off of the Earth than are available now.
But I think it is worth pursuing. The real question is whether we as a society have the same willingness to face and accept death in great feats of colonization that our forefathers had. I just watched a documentary on the Pilgrim's founding of the Plymouth colony in Massachusetts. A bit more than 50% of the people who left Plymouth, England on the Mayflower were dead from diseases within one year. Contemporaneous colonization of Jamestown in Virginia had already proven an even higher death rate among Virginia colonists, so the people leaving England on the Mayflower accepted that many or most of them would not survive the establishment of the colony.
If it comes to that, are modern-day humans and our legal and governmental structures ready to support a colonization effort on Mars that involves double-digit, maybe mid-double-digit, death rates among the colonists?