It's not, I don't think so. For instance, if you install Signal on your Android, the apk was signed by Signal, sent to Google and distributed by Google. Google cannot modify it, because Signal has to sign it. So in order to make you install a "malevolent" version of Signal, Google has to collude with Signal.
Of course you can download and install the apk from Signal's website, in which case Signal may give you personally a malevolent version. But you can easily compare that file to someone else's. On Android there are verifier apps that allow you to check that.
Finally if E2EE does matter a lot to you, you can compile Signal yourself from the sources, and even go as far as auditing those sources yourself.
You don't get to do any of that on the web. When you load ProtonMail in your browser, you don't have any practical way to verify that the code your browser is running is the same as everybody else who is running it around that time. You have to trust Proton that they give you code that prevents them from reading your emails. It is better than nothing or course, but it still means that you have to trust Proton.