My "canon" warden (at least for my first DA:I playthrough) made the sacrifice. Not out of altruism, not out of a fear of what the OGB might become, but out of a selfish, cowardly desire for redemption. In forging Ferelden into the weapon she needed to defeat the blight she had commited so many wrongs. She did what she had to. And it worked. But despite that, she could not live with what she had done. The lives thrown away, the futures sacrificed, the friends betrayed, all to give her the advantages and tools needed to defeat the darkspawn. She had the courage to fight the blight, but she didn't have the courage to face what lay beyond or to live with her sins, no matter how justfied they might have been. So she refused Morrigan's offer, and then, with the Archdemon lying wounded, she refused to grant Loghain his chance at redemption, for, in a last selfish act, that was hers alone to claim. Ferelden had it's Hero, her methods forgotten in the celebrations of victory and she had the freedom of death.
My "second most canon" warden didn't, instead letting Alistair get jiggy with Morrigan. Don't think any of my other ones made the sacrifice either - though a couple had Loghain do so (which I personally regard as the all round "best" option, with hardened Alastair ruling alongside Anora).