It's not "key"; it's not even relevant ... the proof will be in the pudding. Proving a priori that some outcome is possible plays no role in achieving it. And you slid, motte-and-bailey-like, from "know" to "some clear indication of possibility" -- we have extremely clear indications that it's possible, since there's no reason other than a belief in magic to think that "biological" is a necessity.

Whether is feasible or practical or desirable to achieve AGI is another matter, but the OP lays out multiple problem areas to tackle.