Coffee mugs should have an alarm for when the coffee is about to get cold. I'm too focused on work to care about the coffee.

Real.

Solution: Condition yourself to enjoy cold coffee.

Solution: don't work until the mug is empty.

I just reheat. It works

The mug is two times bigger than necessary.

I'm into microcontrollers so this is kinda viable idea to make cup warmer that detects the temperature of the cup, when it goes down it turns on, when hot enough it turns off.

The question is: "Would people pay for gadget like this?"

And it probably already exists...

Not sure it still exists but there was at one point a widget for the KDE desktop in Linux to stop you forgetting your tea/coffee.