Yeah, it's the same thing I've done, after the first week when I downloaded the stuff I had bought on Amazon I deleted the network connections and put it in airplane mode, and it has stayed that way.
But I'm weird, I just want it to stay the way that it was when I got it, and I don't think that I will need any updates, it's there to show text, and it does that well enough, so it doesn't get to update.
@sotolf @afewbugs Yeah, I've got, not the first ever model, but the first cheap-model one that dropped the keyboard. It's been in aeroplane mode for over a decade at this point.
TBH I've never managed to be a big ebook person, but the ones I have bought since then have been indie stuff from Smashwords via Calibre and a USB cable. If I want something from a big publisher I still just buy paper copies.