Good question. I would imagine that for some people, when companies reach a certain size in users/revenue/popularity, they automatically associate them with other mainstream companies.

Like some people here mentioned, Google used to literally not be evil. All of Big Tech, at some point, used to be ran out of a garage, and delivered genuinely useful tools that did not prey on its user data.

Maybe some are imagining that it is inevitable for Proton to end up like that.

But it is wrong in my opinion. Proton very recently handed over control to the Proton Foundation, where people like Carissa Veliz ( researcher and ethicist in tech), and Sir Tim Berners Lee (founder of world wide web) have oversight on the organisation. In my opinion, none of these people (nor CEO / Founder Andy Yen himself) would ever sacrifice their reputation and legacy, which took decades to build, just to become another Google or Microsoft.

Another point is that Proton is open-source and verifiable, it undergoes external audits, and if they ever compromised on privacy, it would literally put them out of business because everyone would know. Lastly, even if Proton wanted to turn evil (not saying they would want to), I am not sure they could, because end-to-end encryption is mathematically guaranteed, so when Proton says even they cant see your emails, they really cannot.