There is a limit on the efficiency of those machines, just like with every other chip we have right now, it can't go and double efficiency every year, pretty soon it won't be happening even every 4 years, the S7 was using 28nm chips, the S19 7nm, there is a limit to how much you can get out of it.
You think there will be a technically limit how fast to solve the SHA 256 function. Dont believe so.
If the profitability of mining is in a lower one-digit number there will also be no more gear sales as the difference in efficiency will not be able to make the difference from the cost of a new gear ordered, thus lowering both the demand and incentive to develop new miners.
What I have learned in my years here in Crypto: As beginner I thought Hashrate and price has a function. But it has not because people speculate (see point 3)