If by "mainstream E2E messenger" you mean WhatsApp and Facebook Messenger, sure. But I didn't realize those were the benchmark these days.
On that note, Signal wouldn't even depend on Intel SGX for security nearly as much if Signal PINs weren't user-chosen but instead auto-generated with enough entropy. Yes, contact discovery through phone numbers would still be challenging, but secure value recovery[1] just requires a key with enough entropy.
[0]: For the record, Threema doesn't store your contact list server-side, unless you explicitly opt in. Similarly, now that Signal supports usernames, my understanding is that one could use the app without uploading one's contact list in plaintext.