See, that is strange to me. Sword & sorcery heroes aren't known for abandoning the woods, ever. I doubt people would have thought much of Conan the Cimmerian settling down with one of his sweethearts at the end of his life.
I agree, a relationship can't change a lone wolf archetype like Geralt. It's obvious CDPR was either cut for time or just wanted to make the ending as casual as possible, last act being the huge mess it was even with Toussaint's lore-breaking ribbon wrapped around it.
He almost did, then Rivia happened.
And thank god for that, a pitchfork in the gut beats killing a character through marriage.