Sure, but all this misses my point that these are comparisons. This is what has happened with other technology in a similar ballpark of "we know how to do this, we can (technically) do this". You're arbitrarily selecting for a specific outcome in the space of all possible outcomes. When not only do we not know enough about any outcome to favour it, but we have knowledge of how other technology being developed in a similar space, with similar constraints, turned out -- i.e. it didn't turn out at all how we expected in any specific way".
So let's be honest here, in the space of all possible outcomes -- the negative outcomes where "it doesn't work" or "it works but is too slow to use" or "it doesn't understand how it works either", is a larger order of infinity than the outcomes where it does work, at least based on what has happened with pretty much every other technology that we have records of predictions for (Is there a technology that we have predicted where it wasn't wildly different or handicapped compared to our dreams? I'd honestly like to know TBH).
There is literally no reason for pontificating more over this. All of this, the best guesses, the worst guesses, is all just a variety of wishful thinking. The only true answer is we have no fucking idea.
First, what makes you think that solving the Collatz conjecture is easier than AGI? Doesn't that beg the question? Just because a challenge is easy to state doesn't mean that we should expect it to be easy to answer. Fermat's Last Theorem was easy to state; open a random math textbook for many theorems that are harder to state, but easier to prove.
Second, quantum mechanics is totally amenable to simulation on a Turing machine. In fact, all of known physics is. You rightly point out that the question is rather how good modern hardware and software is at the task. (Btw, quantum computers would be really good at simulating quantum mechanical systems. And that would probably be their main use; right now we don't really know much else they are better at than classical computers.)
Third, you bring up the point that figuring out how arbitrary proteins fold is still rather difficult. I agree. So an atom for atom simulation of the human brain would presumably be also rather difficult.
That doesn't mean that AGI is impossible. It only means that AGI via atom-for-atom simulation of a human brain would be rather difficult.
Simulating a human brain atom-for-atom is only one approach you can try to take to reach AGI. There are other approaches people are working on. Only one of them has to work.