That's a great insight. We've been planning on making our slack public soon and also keep telegram going. With slack - because security breaches/hacks are so easy and common - every user would be required to use 2fa, which could potentially cut down on trolling as well? There's different considerations to both and we're currently weighing them.
Back in 2014, I was one of the first to use Slack for coin communities. It was obviously a lot better than IRC. It was also great for dev teams to collaborate and chat on with all those plugins. However, like I said, you can't search back on old conversations (3 days was already old when you have 1,000+ members).
If you're gone for week, you can't backread anymore. Whereas in Telegram, even with the busiest of chat groups, you can always read them all.
I think it's best to use Slack for actual developers who are collaborating and Telegram for the general users.
RE: Encryption