Not to mention that those devices all support regular EPUBs out of the box, and so you can still put new content on them today.

Of course, you'll get a bit more out of them if you convert your EPUBs to KEPUBs with Kepubify[0], but the point remains that Kobos are supplemented by their cloud/connected features, not inherently dependent on them.

0: https://pgaskin.net/kepubify/