for me personally I don't care about it on my phone either way, I do on my laptop/desktop

If you have no personal objection to Apple making their products more open, why defend their 'privacy' facade? Have you missed the last major iPhone exploits?

So again, how does an option to open the device affect you in any way?

A big part of what makes the Apple ecosystem so compelling is the consistency and coherence.

Nothing would change for you though, if someone wanted to sideload something they would just have the option to do so.

That's just not true. Making some things impossible by a company with Apple's clout prevents the exploitation of people who will click "next, next" to disable protections without really understanding the implications.

If you're truly a developer, you can add whatever apps you want to your phone from source via xcode, free now, so what's the issue?

Not having the option forces companies like Facebook to play by some simple rules.

Having the option would mean that many apps that are currently available through the App Store, and therefore broadly trustworthy, would suddenly become side-load-only and follow basically no rules at all.