...and there are others that would buy BTC if it was the only option but would be happy with ETH given that it exists. With those people buying / minting X instead of Y (but would use Y if X didn't exist), the amount of digital coins increased out of thin air just like that! Without any control mechanism from the bitcoin protocol (an unsolved / unsolvable(?) problem).

So the fact that the people like you say exists does not prevent the existence of the people that I am talking about.

And if the secure, easy to use, non-wasteful, programmable and distributed ledger problem is solved once then ALL coins after that become the same. In that environment "people who would buy ETH, but never BTC" will not make any sense.