[noobermin]

So, as nickelpro states, I feel like a QC that is actually general purpose (which is probably a better way to state it) is so difficult at this point to even imagine, it's hard to say it will become a thing absent some breakthrough that is hereto unknown. You probably could state it as an energy cost thing by somehow deriving how much energy it would take to keep millions of quibits from decohering by extrapolating from how much energy it takes to keep a few from decohering, but I'm not even sure you can extrapolate that far out since as you increase the number of quibits the required energy probably isn't linearly related to the number of quibits but it is some power law or worse. Remember, the number of quibits we can run is in the dozens today, the numbers you need for Shor's algo or just general purpose computing is likely in the millions.

For quantum people, the QCs are already pretty cool because they can do simulations of quantum systems like molecules and atoms that are just infeasible on classical computing (high performance computing, ie. supercomputer) systems, things that would take probably years (yes years) of wall time on a HPC system. The thing is the number of required quibits for modeling these types of molecules is likely in the dozens to 100+ quibits, which looks possible now since there are systems out there that, while noisy, do have dozens of operating quibits.

If you're curious what these simulations are for, it's doing things like calculating energy levels for certain molecules, which materials science people care about and will help them make the next generation subtrate for a computer chips, etc etc. So it's not entirely esoteric stuff, it will be things which will eventually make it into actual products and technology people use, but it definitely is NOT general purpose computing, even less so Shor's algorithm or breaking encryption.