each one is progressively more complicated than the previous one. So the "price" of "mining" a "bitcoin" is always increasing
This is false. The cost of mining BTC does not increase by default. The cost and difficulty of mining it reacts to the amount of computing power which is dedicated to the network.
Basically, the network attempts to adjust so that no matter how much computing power is dedicated to the network, a block is mined approximately every ten minutes. The way that it does this is using difficulty - every 2016 blocks (approximately 2 weeks) the difficulty of mining a block changes based on how much computing power is dedicated to the network so that a block is still mined every ten minutes.
If the computing power decreased, so would the difficulty of mining BTC.
Today, bitcoins are "expensive", but if the marginal cost keeps going up, shouldn't they keep getting more expensive... by design ?
No. The cost of mining BTC is designed to follow the price, because all investors are attempting to make a profit. When the price rises, so does the difficulty, and when it falls, so does the difficulty.