I think the expectations of e2ee have been greatly stretched in this case. e2ee means that the data is encrypted from device to device only and that's it, from one end to another end. If someone backs up their device in an unencrypted way then thats out of scope for WhatsApp - that's not what e2ee is about.
People that expected full at rest encryption (which is what a backup system would include) despite the app never being advertised that way would have always needed a large kick to realise that isn't the case. Encryption is complicated and you can't expect everybody to fully understand what e2ee/at rest/etc really means. This whole situation is a learning experience for everyone and I wouldn't blame WhatsApp for it either. They now know that advertising encryption needs a little more explanation.
People that expected full at rest encryption (which is what a backup system would include) despite the app never being advertised that way would have always needed a large kick to realise that isn't the case. Encryption is complicated and you can't expect everybody to fully understand what e2ee/at rest/etc really means. This whole situation is a learning experience for everyone and I wouldn't blame WhatsApp for it either. They now know that advertising encryption needs a little more explanation.