Well, to me, Adam Back is not a random guy who doesn't know what he's talking about. Perhaps if there is serious progress in quantum computing, everyone would need to more their assets. To be honest, though, I assumed that if we ever get there, there would be some kind of soft fork rather than individual decisions to switch.
Also, 'maybe 20 years' is very broad and can mean 'never'. People were supposed to go to Mars, grow human clones and many other things in similar timeframes. An answer like that means we're very far from this matter being a serious threat.
I agree that Adam Back's perspective is probably more relevant or interesting than someone here who makes perhaps 50 cents a post declaring the whole discussion to be "low value." I think with the technological singularity near, and with how fast time can move - Bitcoin was only created about 17 years ago - we will likely have to face this issue relatively soon. 20 years is nothing really.
Considering that 1,000,000 BTC is likely more than some of the most powerful nation-states will ever have, it will be interesting to see what happens, since this can, perhaps, be viewed as a national or international security risk. That much BTC in the hands of the wrong person will pose a serious problem.