I think there's little to no chance that you'll convince people to stop using Telegram.  I must admit I find it convenient, quick, and effective to communicate with users I meet here, especially if they're new and have PM restrictions.  Without getting into the whole "end to end" encryption debate, or their willingness to allow scammers to abuse their platform, I just think it's a tool.  A useful one that needs skill and understanding to use correctly.

However, I have never, and never will complete a transaction solely on Telegram.  When I was offering escrow services, all official addresses I provided were posted here, on the forum, either in public or through PM, and always included a signed message.  If all of us bitcointalk.org members would insist on transacting this way and this way only, the Telegram scams would be a thing of the past.