Millions of blocks have been mined into the blockchain by bitcoin miners since the currency's creation.
This is
not true at all. As of the moment that I'm writing this post, there have only been 770222 blocks mined into the bitcoin blockchain since the currency's creation.
centralized fiat money, which is also decentralized.
Not only is this
not true, but you contradict yourself by both calling it centralized fiat money and then immediately saying it is decentralized. Fiat money is NOT decentralized.
History indicates that the first block was mined almost instantly because it had a difficulty of 1.
This is
not true. Even at a difficulty of 1 it likely took a while for the first block to be mined on a CPU. It was certainly not "instantly".
According to experts, the last bitcoins will be mined by 2140.
This is a bit misleading. It's impossible at this point to predict the exact date (or even the exact year) that the last fraction of new bitcoin will be mined. There's too much variability and too much time. It will be somewhere around 2140 approximately, but it could be anywhere in a range of many months (or even a few years). It is entirely possible that it could be a year or two earlier, or a year or two later.
We still have a long way to go before we can completely mine the entire bitcoin blockchain.
No. There is no "entire blockchain". New blocks can always be created, forever (or at least for as long as people want to continue to use bitcoin and create blocks).
