Why Quantum Computing isn't realistically a threat.

So I had a coworker ask me about the threat of quantum computing on the security of bitcoin.

It got me thinking about it further. The main reason I've seen floating around is that building quantum computers is hard, and the technology is far enough away not to really matter.

Going in a bit deeper on why this is so, not only would someone need the resources to build one of these special-purpose machines, they would need multiple of them under their control.

Since mining is just rehashing entire transaction history every block, they would need to put transactions, or lack thereof, into new blocks, and be able to solve it first. However, for it to reach enough consensus, multiple miner nodes would need to confirm it, so the speed of solving each block will still only be as fast as the remaining miners solving the same hash.

Then, say an entity has put forth the capital and other resources to do just this. What incentive do they have to justify that effort? This entity spends hundreds of billions of dollars, possibly trillions, to create machines to fully solve bitcoin and mine all of the remaining coins. If it happened today, they mine >1 million coins. Well, total supply can only reduce going forward. As long as overall confidence hasn't dissolved, Value of bitcoin will then only be determined by the demand and security. And if overall confidence dissolves in it driving value to 0, this entity has now wasted all of the resources building the quantum computers that would need to be repurposed to do something else to remain useful to the entity, otherwise it's, purely wasteful.

Thoughts, or better reasons why quantum computing isn't a realistic threat to bitcoin?