This is such an underrated feature. I used to own a Kindle before, and now a Kobo Libra. I'm very pleased and satisfied with the Kobo - something I rarely feel about consumer devices these days. Kobo should be proud of themselves for sticking to the principle. I will not spend my money on anything less open when it's time to replace it. I hope the vendors take note.
Freedom and openness should be considered as a feature for any product - perhaps the most important one. And us, consumers need to encourage and if possible, force the vendors using our collective purchasing power, to offer us that feature. I may be preaching to the choir here. But this message is well worth spreading among the public. Please do.
PS: I have seen DIY devices that are more open than Kobo. But Kobo is also the most viable option here. Please mention any alternatives that you know of.