Based on my understanding of the purpose of the Individual Mandate is that it's less that the well employed are subsidizing the under/unemployed, and more that the healthy are subsidizing the unhealthy.
With everyone left to their own devices, the elderly/unhealthy are strongly incentivized to have insurance, and the young and healthy might reasonably conclude that their best bet is to not pay for insurance, that results in higher average premiums.
People waiting until they get sick to sign up, essentially. The insurers' previous solution to this was no coverage of "pre-existing conditions". But people hated this generally, for various more or less good reasons. Banning exclusion of preexisting conditions is one of the handful of parts of Obamacare that's wildly popular across the political spectrum, at least among non-politicians (it polls something like 70-80% support). Obamacare's main replacement for that is the individual mandate requiring you to always have insurance, rather than to go without until you get seriously ill and only then purchase it. (There are also a few secondary things to try to discourage buying insurance only right before you're going to use it, like the open enrollment period being only once a year.)
