Again, you are questioning reality with your imagined scenario. If it was indeed profitable, it would be in mass usage by now. The REALITY is that it is NOT profitable to run a ASIC as a heater, because it CAN NOT be turned on all the time, therefore the hardware cost can not be easily recouped. It has no hope to compete against a full time mining farm.
Some heaters are on all the time, but lets assume that we have intermittent Asics competing with full time Asics for a moment.
Option A-
ASIC hot water heater running 20% of the time cost efficiency = (amortized costs of design, testing, build, and shipping / millions of units) - 20% normal rate of transaction fees and block rewards compared to standalone miners
Option B-
ASIC Standalone miner running 100% of the time cost efficiency = (amortized costs of design, testing, build, and shipping / thousands of units) + electricity costs + possible costs for cooling
Do the math . What is more efficient?
P.S... remember in option A electricity isn't included because its a needed product already consumed.
They can not be on all the time, why would you have them on when the temperature is above 76F? Waterheater can not be on all the time neither, they run in cycles, otherwise you are just wasting water/energy.
The problem is when you can't have them on near100% of the time, you could never recoup your hardware cost, because like you said, new ASIC will be come cheaper and more efficient, therefore driving up the difficulty, and your old hardware, when not on 100% of the time, will never recoup the cost. Which makes your ASIC part of the investment a loss when competing against a real mining farm. So you basically bought a very expensive space heater/water heater, when you could have gotten a regular one cheaper, even considering the total cost of ownership.
Again, like I told you weeks ago, when your imagined scenario doesn't exist in reality, you need to first question your scenario, not question reality. Also again, if you truly believe in your scenario, then build the product, and let me know of your huge success.