I would refer to the recent FTC questions to “algorithmic pricing practices”. Long story short it was about some companies are using browser data, accounts data, etc. for “smart” pricing. Your brother may not care about someone watching him but I don’t think he wants to pay for the same goods more than others.
I heard that in USA Telegram is used mostly by conservatives, trans-haters and drug dealers. But in Russia where I was born and where I have been living until the war, Telegram was a single source of alternative news and the main tool for opposition to putin. I had a talk with an Iranian guy recently and he told me the same: in Iran Telegram is the only source of non-state-propaganda news.
I do not care that Durov is just a billionaire who makes money on the trend of privacy. But I remember how Telegram successfully fought against attempt of blocking in Russia in 2018 by providing regular updates and using different techniques to avoid blocking. An attack on Telegram for me is an attack on the latest opposition to putin, khamenei, lukashenko and others.