I can also imagine that the microchips and their connected components wear out faster because they are under high load and therefore become hotter than usual. So after a few months your fridge is mysteriously dead just because of malware and you won't even know about it.
Imagine an army of fridge-hosted bitcoin miners…it'd be extraordinarily inefficient, but the bot-herders wouldn't care about the electricity bill.
Are there consumer routers (that are relatively easy to use) that can alert you or shut down a device, when all of a sudden your thermostat is making tons of network requests?
There are routers that can control how much data a given downstream device can transfer in a given time, shut them down entirely during certain times of day, etc.
But then I'd guess the crooks will just compromise the router itself.
And the average IOT user is unlikely to understand the security implications and correctly configure the router.
But would you notice if your thermostat was running a little slower than usual? Would it even occur to anyone that their dishwasher might have caught a virus? These things could participate in DDoS attacks all year long and nobody would suspect a thing. And even if someone did, fixing them would be much more difficult than taking your laptop to the nearest Best Buy (or even impossible, thanks to DRM).