The strong expert consensus has been reached between a lot of people who would be out of a job if their consensus was the opposite.
https://www.quantamagazine.org/the-argument-against-quantum-...
In any case, I'm happy to bet on this.
And which odds would you need on that?
If you wanted a useful machine, say factoring RSA-2048, you would need to push the date to more like 2050 or 2060.
Most of the uncertainty comes from the economy, engineering cost, public interest in QC, AI doom, etc. If we could somehow make a platonic bet on “is a large QC constructible if dedicated $10T/yr for 200 years”, then I am over 95-5.
Not to mention the fact that we still don’t know if the current candidate PQC protocols are actually secure. Security is mostly a game of back-and-forth over years, so it could take a while.
As for my own belief, I don't know how it will pan out, but I'm inherently sceptical of people who are too certain that it is one way or another, the hard evidence simply does not exist yet.
I'm personally most inclined to believe that the universe is computationally classical, and while quantum physics necessitates that there must be quite a lot more information and processing happening than a Newtonian model requires, it is still a finite, thus putting a finite limit on what speed-up we can gain from a quantum computer. Most likely this pans out as the error correction limit being impossible to reach, but there is also the possibility that more complicated quantum states result in higher per-gate error rates.
I'm open to other conjectures, but if you want to dispel mine I would like to see some hard evidence.
New fundamental physical principles are at this stage required in any case, quantum mechanics and general relativity don't fit together, meaning that at least one of them need to be overhauled into something that explains the same observations but doesn't have quite the same maths. The lacklustre performance of quantum computers could be a hint to what needs overhauling.