Still avoiding the main point - e2e encryption does not prohibit syncing, backup etc for said data (without decryption)
Telegram is all about convenience, security is just bolt on. Everybody says they care about security, but hardly any TG user does it, because it is inconvenient. Install it on any any new device, confirm your phone number with an sms and whoops, all your chats and drunk and stoned pics are back.
But the WhatsApp iOS client backs up to iCloud, and the Android client backs up to Google, and this creates a blocking issue unless the user is willing to jump through hoops and use 3rd party tools.
It's already out there, just a matter of adopting it.
When you use Matrix, and open a web client, the only thing you have to do is 1) logging in and 2) providing a password, key file, scan a QR code, OR compare Emojis - and you get everything synced.
The same holds for any type of client. I barely see a loss of convenience, let a lone something being hard or impossible.
Signal seems to have been around for ages. OTP even longer
Older, Element (also known as Riot and Vector) should already have been existing for at least 5 years.