The idea has been around for a long while. Google "data furnace".
The crux seems to be Moore's Law. We buy heaters and use them for decades, but any compute device has a useful lifetime of a few years before it's surpassed. If we ever see Moore's Law come to an end, expect to see a lot of highly-distributed computing.
Under these conditions, it could still work in irons or especially electric kettles. Those are so crappy nowadays, you have to buy new every two-three years anyway.
Unless the device is on almost all the time, it won't compete with datacenters, even on price.
Using compute for heat is a dodgy proposition even where heat is needed at least half the year. The minimal uptime of an iron or kettle really puts the kibosh on the economics.
The crux seems to be Moore's Law. We buy heaters and use them for decades, but any compute device has a useful lifetime of a few years before it's surpassed. If we ever see Moore's Law come to an end, expect to see a lot of highly-distributed computing.