The blight has the same amount of chance of killing the Warden too. Also the warden can still hate their situation too, they don't have to like joining the blood brigade, but they have a responsibility at the same time. The only reason why the Warden is alive at all is due to Flemeth 1: saving the warden and Alistair and 2: while in the wilds she shielded him/her from detection since you are a new warden. When you reach Lothering you see the effect that this blight is having on the people and you discuss how there is no way to get outside help in time to prevent whole sale slaughter (You still don't but it's no where near as bad). Also when you reach Lothering you are told about the bounties put out on wardens and how they supposedly "betrayed and killed" Calin. At that point you also have a target painted on your back no matter where you go. At this point the only debate is between who should lead the group and it is deferred to you, the warden, because Alistair is afraid to. So at this point you seem to be in a damned it you do, damned if you don't situation, but if you try to use the treaties you might have a chance to save thousands of lives. I never once saw how it was possible to just walk away. You become a hero, because no one else can fill the role since Alistair abdicated the role. He dropped it on your shoulders to be the one. To flee would doom thousands to death. I'm not sure about you, but that's not something I could ever live with and very much so does not make a "Mary Sue"
Refer to my comments above this one for why it's all but spelled out why you can't leave. May I ask exactly how you felt when you found out Loghain wanted your blood and all wardens dead? And wether you wanted to be a warden or not doesn't change the fact that you are one. Also in Inquisition you pretty much are sacrificing yourself at Haven, there is no chance that you will survive it. You have to play the role of hero from the very beginning pretty much.
Well, I never said anything about a Mary-Sue, and again, there are plenty of reasons to stay. Not everyone is a hero, however, and while you and I wouldn't choose to leave, there are plenty of people who would. Furthermore, while you and I know that nobody else can fill the role, the Warden might not. Like I said, they might consider Ferelden a lost cause after Ostagar and flee to Orlais. Maybe to save themselves, maybe to warn or even join the Orlesian Wardens.
As for leaving Ferelden, the fact that Loghain wants you dead seems like all the more reason to get out. His resources outside of Ferelden are likely far less than inside, and if you're not around to cause trouble he could well decide to focus his efforts on the Blight instead.
We can't all be heroes, and in realizing we can't save everyone discretion may become the better part of valor. Abandoning Ferelden to make a stand elsewhere seems like a realistic line of thought. Whether or not it is the optimal choice or the morally correct one doesn't have to factor into it. Your reasons for the Warden staying are perfectly valid, and as I've said before, match what my personal decision would be (at least I hope so). That there are valid reasons to leave as well doesn't really seem like it should be such a point of contention.