Infrastructure is a lot more complicated some pressure capsules and solar panels. Infrastructure to make a colony viable would mean agriculture and industry (including ways to deal with their negative externalities). Everything about both of those would need to be bootstrapped from Earth.
Even at SpaceX's best rates for the Falcon 9 and Dragon capsules at maximum capacity it would take over 14,000 launches to put those 100,000 colonists into orbit. That alone would cost a trillion dollars (assuming awesome rates from SpaceX and no failures). Just the structures and resources to keep those people alive for the first year would cost several tens of trillions of dollars more. The infrastructure to make an actual colony...well hopefully you get the picture. To put the numbers in better perspective we've only launched a little over 300 manned orbital missions in history. Ever.
A solid outline of the challenges, but a simplistic understanding of the proposed solutions. You don't put 100,000 colonists in orbit as fast as possible and put them to work building a metropolis on Mars. You put a few fertile and healthy couples on Mars at a time, over and over, and you grow the colony over hundreds of years. In fact, you grow several independent ones simultaneously, so one can evacuate to others in the event of an emergency. An additional 300 launches spread out over, say, six Earth-Mars launch windows (about 10 years) could mean 1200-1800 colonists, not counting any children they may have, and the infrastructure, agriculture, and industry involved in supporting those people would grow with the population. It would be heavily reliant on Earth for a long time, but that reliance could slowly disappear over time as Martian humans make do with what they can produce themselves, which could end up being a lot more than we can imagine with today's technology.
Of course, also on those timescales (centuries), the ability to recolonize Earth is questionable. Even the first human that grows up on Mars would have a great deal of trouble adapting to Earth, which would have three times his or her native gravity! They could probably do it with some fancy exoskeleton technology that might not exist yet, but their bones wouldn't be used to the stress at all, and they could easily have some crippling agoraphobia. But who knows what other adaptations the human body will undergo, given enough time. If anything, colonizing Mars is a stepping stone to a mostly space-faring civilization, but that's far, far into the future.