This is an interesting concept but doesn't seem like it has long term legs.
It depends on what you mean by open source and also even eBook reader (I'm assuming eInk), but if people want open source e-readers I would say flashing existing reader hardware with open source operating systems would be the way to go. However I'm not sure if there is much motivation to do that.
There are Android based eink ereaders available with more freedom than Kindle devices (Boox is an example) and you can side load free or open source reader software onto Kobo (maybe not Android Kindles though?), and you can load free books onto e-readers via software like Calibre. So you can read books in privacy outside the vendors ecosystem - it kinda reduces the imputus to build an open source ereader (hardware or OS).
I'd love to see a truly open source Eink device - particularly software wise. But I doubt the demand is enough. And this Open Source hardware solution seems a bit too cut back to fit the bill.