the bitcoin-miner-heating is competing with other forms of resistive heating which are way cheaper
No. A $200 S9 (which could fall further ... maybe back to early 2020 pricing of $50), which is essentially equivalent to a very loud 1,400W space heater, produces income. Maybe $30 / month at the current difficulty and exchange rate.
Bitcoin mining for heat won't work for everyone, but it can work for many.