@feld i know this source is old but just look at the this 2017 blog article by Pavel Durov: https://telegra.ph/Why-Isnt-Telegram-End-to-End-Encrypted-by-Default-08-14
it immediately opens with the most far-fetched and niche argument ever. All of E2EE is not what you truly want because... some users to unencrypted backups.
The same users would do the same backups for telegram but let's ignore that.
All of Telegram shall have no E2EE because they leak it anyway.
Signal is niche and cringe (didn't use the latter word) because they encrypt backups