It is basically a hole in a bucket. As long as money is going in faster than it is leaking out it will work. The overall medical cost of people who could afford insurance but are not getting it then get something like cancer is a pretty small percentage of the over all medical cost in this country and it won't crash the system. The mandate was at bone they threw to the insurance companies to get them to sign on and was more about forcing young healthy people to subsidize older less healthy people than it was about pre-existing conditions.
With no pre-existing exclusions, a mandate to purchase is the only thing that stops everyone from waiting until they are in the back of an ambulance to get insurance. The provision is essentially there to keep the insurance companies from challenging the entire ACA in court (and winning) under restraint of trade and them not being subject to the interstate commerce clause as they have in the past. The most recent SCotUS precedent on the topic has insurance being State regulated.