[Canada]

First of all, Telegram is not end to end encrypted. It has limited support for end to end encryption of direct messages only. There is zero support for E2E of groups, and zero support for E2E in a multiclient context, which makes the feature almost completely useless, and indeed, almost nobody uses it and I don't think even 1% of telegram users have ever used it or even know how to use it.

Integrating either Signal or WhatsApp clients with Telegram would to tantamount to a backdoor, or would require a redesign of Telegram so drastic that would upset its users. A perfect analogy is integrating WhatsApp and Twitter because Telegram is actually more similar to Twitter than it is to WhatsApp.

But even integrating Signal and WhatsApp which now share a similar encryption scheme and user ID isn't a good idea: Most Signal users don't want any interaction either directly or indirectly with WhatsApp. The two programs have completely different group implementations: WhatsApp keeps all the metadata server side, whereas Signal handles all of that client side with the server acting as a dumb router of opaque messages that wrap the group state updates. How would those two systems be reconciled? Would Signal have to accept WhatsApp users joining its group and then just leak all of the group metadata to WhatsApp servers for compatibility? Would WhatsApp have to re-write its entire service to be compatible with Signal's group semantics and see to it that all billion users are fully upgraded or kick them off the service?

These issues go on and on and on, and they increase exponentially the more messaging systems you try to add. The whole idea is just ridiculous.