Can we be real for a minute though?
It's still better than not having it as an option.
By telegram existing, it diversifies the non private messaging landscape. It's obviously not better than actually secure and/or private services, but the more options that are out there, the less centralization there is, which is a net positive.
You just have to be aware of its limitations and don't use it for anything significant. In that regard it's no worse than something like discord.
You already covered the warnings about not trusting it for privacy or security, so that's really the beat you can do in informing people. Once you've done your due diligence for the people you care about, you gotta let them do what they're gonna do. It's either that or go hard and refuse to communicate on anything other than the services you deem best for your preferences and hope for the best