This is more from Schneier's book, but I would say the most import reason E2E encryption should be the default is that in the event of a data breach, nothing would be lost. If a company's servers are hacked, they'd have access to the symmetrical encryption keys, and therefore all of the data. It also ensures that the company can't be selling/sharing your data, as they don't have access to it in the first place.

Edit: I also meant iCloud backups in my original post and how Apple can decrypt your E2E encrypted iMessages with the key the backups contain. But I posted it last night and couldn't edit it once I caught the error. It would be amazing for other iCloud services to have E2E encryption so long as the implications of iCloud backups having your encryption keys is stated front and center when choosing to opt-in.