I don't see a problem. When you look down the iron sight or scope you are totally focusing to the crosshair whether you are zooming (FOV changes) or not. That is how it is in real life and that's how it should be in games. If using scope/iron sights was easy and didn't have a downside, people would always use them. That doesn't happen IRL and it doesn't happen in games either, and that is successful game design in my opinion.

I don't think this is even matter of realism or not. Aiming accurately simply needs a tradeoff and leaving it out would not "fix" anything. It's the same as you need ammo to fire weapons, you need mana to use spells, you need something to step on in a platformer game etc.

So if you have a scope, put a zoom with field of view change on it so you see the smaller area larger because field of view changes. You can even make the scope fullscreen like here so that nothing besides screen overlay (scope shape) and FOV limits the view. If you use iron sights, make the weapon more accurate and put a small zoom / DoF effect there mainly to indicate this to the player.

BTW If you think the image you found is bad check these examples from Day of Defeat Source:

http://i.imgur.com/HR2P8.jpg vs http://i.imgur.com/S4NcD.jpg

But the only thing that is off about the above examples is that they are supposed to be corresponding weapons for opposing teams. Did the developers take into consideration that the first has much worse aiming in iron sight mode as they balanced the weapon damages? (the first one basically does double the damage but is also slower, smaller clip, etc) I believe they did.