Insecure by default is the same as insecure.

Imagine if your internet banking worked the same way. By default anybody can take money out of your account. But if you jump through some hoops you can set up authentication.

Nobody would accept that and rightly so because it is the default security level that matters not the highest one.

WhatsApp is much more popular than Telegram and it has E2E by default, so any argument that users won't accept E2E by default doesn't hold water.