Do you think there is a significant chance that quantum will never take off (i.e. there are non-obvious limitations that will prevent quantum architectures like superconducting qubits / trapped ions / quantum dots /... from ever outperforming classical supercomputers)? Related, what in your opinion is the best indicator (or would be the best indicator if demonstrated) of the potential of quantum devices?
The depressing possibility, of course, is that we never succeed in building useful QCs, but we also never learn anything deep about why we failed: it was just too complicated, too messy, and then at some point the funding ran out.
But I like to proceed on the assumption that the world is ultimately comprehensible (what other choice does one have in science? :-) ). On that assumption, if QC can never work, then there must be a deep reason that's not articulated in any of the existing physics books: either a breakdown of quantum mechanics itself, or else some new principle on top of QM that "screens off" or "censors" QC. Needless to say, the discovery of that principle would itself be a revolution in science -- indeed, I'd personally be more excited about it than a "mere success" in building scalable QC! (But my own bet is on the "boring, conservative" possibility, that QC can ultimately work.)
If we see the milestone of "quantum supremacy" achieved in the next few years -- i.e., a 50-70 qubit quantum computer used to solve some artificial sampling task many orders of magnitude faster than we know how to solve it classically -- that will obviously be one strong indicator that the potential of QC can be realized. An even better indicator would be the use of a quantum error-correcting code, like the Kitaev surface code, to keep encoded qubits alive for longer than the underlying physical qubits are staying alive for (or better still, to perform 1- and 2-qubit gates on them).
By "significant chance" do you mean something like 10% or something like 70%?
Sorry, couldn't resist :-)
Even if quantum computers required near zero temperatures, superconductors and such stuff, there is no reason why you couldn't have all that in a no-serviceable-parts-inside box if the economic incentives were strong enough.