The ACA must have good parts and bad parts because the mandate makes the numbers work. Guaranteed-issue insurance doesn't work if healthy people can opt out (because you can just buy insurance after you get sick which makes the risk-pooling idea of insurance not work).

Well, you can't, because of defined enrolment periods and coverage delay periods, and that it's impossible to tell ahead of time whether given insurance covers a given condition.

In addition, medical insurance makes no sense as a risk-pooling strategy, as everybody consumes some medical care annually, and the average cost of care consumed is greater than the amount most people can pay. There has to be something else going on.

There is, because we live in a finite world. Everything is about trade-offs. In the case of ACA, it’s benefits on one hand, and the taxes (and the mandate) that pay for those benefits on the other hand.