I don't think hackability is so much the point as open ecosystem, privacy, and interoperability. iOS wiped the floor with Symbian, and Android wiped the floor with iOS due to being more open than the predecessor. Linux is the most popular OS kernel in the world (think routers, smart devices, etc.)

I want a choice of thermostat which fits my decor, and for that to work with my smart thermometer. I'd like to buy both on eBay from a Chinese company if I'm poor, or at an upscale designer store if I'm rich.

What exceptions to this rule exist?

Where do we have widely-used hackable devices (or tools, processes, environments, etc.)

I'd include such areas as gardening, cooking, creative activities (photography, music, etc.).

Are there any notable exceptions?

Hrm: arguably clothing and cosmetics. There are people who will spend a non-trivial amount of daily time putting on their face or wardrobe.

This. X10 has been around for decades. Even the most common of devices - the home router - how many run openwrt?

I don't think that the devices themselves need to be hackable so much as the protocols need to be open. Any phone I buy works with any router. I think that's the kind of openness we need.