Expecting someone to remain faithful to someone who died 10 years previous is very selfish.

Things change and people move on. Mourning someone long after they're gone while moving on to someone else is fully possible. You don't have to up and forget those who died when you move on. We continue to remember them and use those memories to fuel us going forward. If we all mourned someone and didn't move past them, we'd never remarry after a death or a divorce. For pet lovers, they'd never get another pet after the first one died. We'd never make attachments because we'd be afraid of losing them. Such is life. We get through it and we move on. Only the selfish wouldn't want those left behind to move on. And I very much doubt the Warden wouldn't have wanted their love interest to remain locked in the past. It's often said that if you truly love something, you know you have to let it go. Move on but don't forget. Don't dwell on the past, move forward. Time heals all wounds. There's so many ways to put it.
Should past love interests be available in DA3? Possibly if they're done tastefully. Just having them pop in for a one-nighter isn't my idea of being tasteful ... well, except for Zevran and Isabela. They should acknowledge they had a relationship with the Warden but also state that they've moved past that. The only time an old companion should be locked out of a romance is if they're still with the Warden (or Hawke). A Warden who married Alistair should lock out Alistair from being a romance option although he could still be available to flirt with, but a Warden who died shouldn't lock out Alistair at all even if he married Anora. Like father, like son. Somehow if Maric or Cailan weren't faithful to Rowan or Anora, I doubt Alistair would be faithful to Anora, either.

Begs the question if he'd be loyal to the Warden ultimately as a result, but this isn't on Alistair anyway. Having old companions moping over the Warden having done the ultimate sacrifice 10 years earlier is also a bit lacking when it comes to the story. From all indications, every companion moved on after DAO so why should they suddenly do a 180 10 years later? Frankly, life's too short. If your Warden died, that's in the past. Let the companions move on and not be stuck in it. If anything, most of them should be married or in a relationship with someone, possibly even have a kid or two by DA3.