Hardware is a high cost if you're using current gen mining hardware to be competitive.
If power is essentially "free" (since you'd use it anyway for heat), then even very old mining hardware would have a net positive benefit for you.
It might only be worth a cup of coffee a week, but it is "free" money you can collect...
It still depends on what your alternative is. Much of the world does not use electricity for heat; in the United States, electricity is rarely used for heat in in the parts of the country that traditionally get brutal winters. It's cheaper for me to heat my home via the forced-air natural gas furnace (even though it is a less efficient fuel-to-heat process as compared to resistance heaters) given the disparity in pricing between residential electric and natural gas costs. I tried mining ethereum for a month last winter and I don't think that I broke even (despite using a relatively recent RTX 2080).
