The physics underpinning fusion was proven in the 1950's. Since then it's been an engineering problem.
The physics underpinning qc was arguably proven in the 2020's. It's not quite done (in the way that fusion was not quite done in the 50's) but there is a fairly clear set of demonstrations that QC's with error correction are possible. However the engineering barriers are fierce and there is still a possibility that they are insurmountable. In addition there are concerns that while QC will work the class of problems that is NP and also BQP may be very small. Even if a problem is in that group then it may be that the algorithms we have are not superquadratic or quadratic - meaning that the improvement that they offer over classical algorithms may be marginal.
Worse, there are often very good heuristic approaches to some of these problems which means that although a superquadratic QC approach would be an amazing breakthrough of computer science (genuinely amazing and worthy of accolades and prizes and fundamentally important for our understanding of the universe etc) it would offer only marginal economic value (possibly). Now, this is not true of some problems where there are exponential explosions and no good heuristics... but there is an even worse catch.. Which is that the quantum algorithms offer computer scientists fresh insight into what's tripping up the classical approaches. In this scenario it can be that an amazing breakthrough happens in QC, and someone uses that to get an insight that pushes the classical approach close enough to the QC approach as to render the QC approach marginal.
The theoretical picture is moving very fast though - so we will have to see.
On the other hand the practical side is moving more slowly. We see announcements that make one think that a Moore's law type of scaling is happening but hidden in the small print there are often (always as far I can decode) catches that mean that while the results look great they are still very much mired in problems. For example, are all the bits on a QC useable at once? Can they be used to form an actual algorithm? How long does the machine run for? How long does it take to start? Some of the answers are jarring - often only a small subset of a machine can be used in an actual problem solving episode; sometimes the machines run for a few steps only; sometimes the machines take 24hrs or longer to start.
It has taken 70 years to nearly build fusion reactors, it took 70 years to create mRNA vaccines. It may well take 70 years (from now) to build practical, valuable quantum computers. And something could go wrong on that path that just renders them moot.