One bitcoin in an address with multiple inputs is not equal to one bitcoin in an address with one input.
Incorrect. One input is not the same as ten. But one bitcoin is exactly the same quantity regardless the number of inputs. This is the same as arguing that ten euro coins can have different value than a 10-euro banknote. They have exactly the same value, it's just the weight that changes.
One bitcoin on a legacy address is not equal to one bitcoin on a segwit address or one bitcoin on a taproot address.
Incorrect. Legacy address is not equal to Segwit address. But one bitcoin is exactly the same quantity regardless the script used. Similar analogy applies here as well.
There is a number of evidence that investors of a certain class prefer to buy bitcoins not on the exchange, but directly from mining companies, and even pay an additional premium to the market price for owning bitcoins with zero history
Please point me to one such case.
Bitcoins from the earliest period of history (about a year after the genesis block) are under close scrutiny by blockchain analyzers, and their transition from a quiescent to an active state can lead to significant consequences. One bitcoin from this period of early history is not equal to one bitcoin from a more modern period of history.
Again, bitcoins are equal. I can acknowledge that the owners of those particular coins need privacy, but the bitcoins per se are no different to "modern" coins.
Because you can mix well, but you can't hide the fact of mixing.
You have fundamentally misunderstood mixing. You're not supposed to hide the fact that you mixed. You're supposed to obfuscate your coins' origins. You pretty much
want from the rest to know that your coins come from a mixer.