Realism and leliana ending yeah... 
Clearly you don't know about Elizabeth then.
Took power from church which was impossible at the time due to sheer power religion had
Had greatest spy network of her time
Perfectly talented at learning secrets and intrigues.
Absolutely crushed the religious sects opposing her, I think Leliana's "grand cathredreal flowing with blood" applies here
Got the support of common people
Restored peace which seemed impossible at the time but no she restored peace with throat cutting and spying
After hundreds of years peace was restored, literature and art flourished even the genres which were forbidden by the church
This was almost impossible to achieve in her age, but she did it and because of her England is still a superpower to this very day.
The point behind this is no commoner will join a rebellion against Divine when the divine is literally their salvation (Which Leliana's epilogue implies she is). The rebellions will solely rely on mercenaries and they will die as soon as they form. It was key to success for Elizabeth as well. You may think Leliana's epilogue is "impossible" but we both know its not impossible, its just unlikely but Leliana has taken the only route with any chance of success, the support of commoners.
Also speaking of realism, Martin Luther King or Anne Frank or Rosa Parks or Gandhi achieved similar goals with Leliana, so yes Leliana's epilogue is unlikely but definitely not impossible.