People keep making this argument - it falls apart as soon as some big-name or currently hot apps decide to make themselves available only on the third party stores, and then iPhone users are forced to make a choice to either, as you say, "continue using App Store and trust apple that third party applications that you download are safe", or start using the third-party app stores where those apps are available - Epic, for example, would do this in a heartbeat, Fortnight would only be on the "Epic iOS Store".
As it stands today, iOS users can count on a lot of things for every single app they download - in terms of privacy of their information, financial details, tracking, etc. Because Apple has standards that every app has to meet, and a whole range of unsavory behaviors that aren't allowed (yes, there have been apps that violate this - they generally get caught and forced to conform, or get ejected from the store). And precisely this has made a whole lot of customers happy and has made them feel confident about downloading (and paying for) lots of apps. Epic, essentially, wants to kill that, just so Epic can get more money. Let's be clear, this isn't about "freeing the users", it's about making more money for Epic, and other big developers.
And, again, if someone wants the wild west frontier of app stores / downloading / sideloading, it's widely and easily available - all they have to do is choose to buy an Android phone instead of an iPhone - they're not hard to find, they're available everywhere. People keep portraying this as offering more choice - it's not - it's taking away one of the two current styles of ecosystem (the "walled garden approach"), decreeing that the other is better and everyone must only have the Android approach.