1- "Mining Involves Computation-intensive Operations" => this is just a cheap brute force loop for guessing the nonce
2- "Bitcoin is a decentralized cryptocurrency" => Whales and Pools can control the price. 65% of Hashrate is concentrated in China.
AFAIK it uses two iterations of SHA-256 to generate the hash.
ie. A cryptographic hashing function designed to be computationally expensive/infeasible to reverse.
For my is just like a brute force password try-loop with a hashing in between. And that is the whole science XD
The above is massively computationally expensive, hence why it's not a fallacy.
That is why you waste more energy (more expensive) now than years ago.
If the problem was adjusted where it wasn't using a double hash, and had some simple two-way function than it wouldn't be considered computationally expensive. That is irrespective of the length of the nonce. E.G if it was just a how much do I add to get a nonce of length x.
The complexity here is caused by the double hash.
The length of the nonce is just an even further scaling of an already difficult problem.