Can’t it be both poorly designed and not explode? There are problems with the ACA. Lots of them. Most are easily fixed (but require a commitment to keep the law in place, so they haven’t been–such as the legalism that allowed states to opt out of Medicaid expansion), and some are truly structural. The ACA is ultimately what you get when you try to let the free market handle insurance with minimal government interference. And that’s not unexpected with its parentage–the ACA was dreamed up at the Heritage Foundation, with Newt Gingrich an early and vocal champion for the individual mandate, and a test-run of the system at the state level under Mitt Romney. It’s the ACA’s quality of bending over backwards to avoid excessive government interference that keeps it from working very well for a lot of people. But it won’t explode.
Yes, on average, premiums are rising much slower than they did before the ACA. Nobody disputes that (outside your standard propaganda mills, using cherry-picked locations). But that doesn’t mean it’s problem-free.