One of the reasons that the whole ACA repeal/replace thing has been such a Keystone Cops event on the Republican side is that the Individual Mandate brings up deep philosophical divisions in the Party that are difficult to bridge.
On one hand, you have the business-oriented conservatives who have an aversion to "free rides" and therefore believe that everyone should be required to have health insurance. This means that everyone who shows up at an emergency room should be a paying patient; no more free care that the hospital has to write off. This also means that there is a larger pool of healthy and sick people paying into the insurance plan so that the fund is solvent and able to provide for those who need healthcare.
And on the other hand, you have the libertarian wing of the Republican party (i.e. Rand Paul, Mike Lee) who believe that the government shouldn't be mandating that people have insurance. The more libertarian (and the Tea Party) wing has tried to weaken the mandate or eliminate it all together. This means that older people and people who have health issues are buying insurance but younger and healthier people are hedging their bets...
...because the penalty is assessed by the IRS and it's something like $700 for the year. So, a healthy 22 year old person who is not on their parents' policy weighs the cost of paying $150 per month ($1800 per year) for health insurance vs paying $700 at the end of the year as a penalty. Many of them may not be aware that the government subsidies would probably cover the monthly cost of the insurance, making their out-of-pocket next to zero.
The result of having a weak mandate and a small penalty for not having insurance means that some people are still uninsured. It also means that when these people need an emergency appendectomy or have an accident that results in injury, their hospital care is free to them but results in higher costs for the rest of us since we pick up the tab for the uninsured's "free" healthcare.
The idea behind the mandate is to get rid of the cost of uncompensated care which was cited as one of the reasons that drives up the costs in the system. It's not something that the Democrats just came up with. On the contrary, several of the hybrid social-private health systems in Europe have an absolute mandate- everyone must have health insurance of some form. It's one of the key pieces of the Massachusetts equivalent of the ACA (aka Romneycare). It was part of the conservative think tank alternatives to the public option (e.g. Hillarycare) in the 1990s.
In another of his continuing ill-informed pandering to the base,
Trump signed an Executive Order his first week in office that guts the mandate.
Until there's a compromise between the libertarians, the business Republicans and the Democrats to enforce the mandate and make penalties more unpleasant, the financial model underlying the Individual Market is going to remain unstable.