It still boggles my mind that people are surprised by this. Mathematically, including people with pre-existing conditions will mean either that those people are required to pay for the (expected, not actual) costs of their own care, or that other people will be required to pay for those costs[0]. This effect is much larger than the effect of mandating insurance for younger, healthier people who otherwise elected out of getting insurance[1].
Note: this is not a normative statement about whether this is a "good" outcome. This is just a a mathematical observation about the way insurance works, from a mathematical and economic perspective.
[0] Subsidies come from taxpayer money, so they have no net effect on this accounting - it changes how the money changes hands, but the money still has to come from somewhere.
[1] I'm waving my hands around the math here, but if people are interested I can dig up the more detailed blog post draft that I did a while back explaining this in full detail.