Denethor was the twenty-sixth ruling steward. The year, when Denethor and Boromir died, was the nine hundred and sixty-ninth of the stewards' rule. Boromir's understanding of the war, the threat, and the enemy was limited.... focused, but limited.
@farntfar just pointed out that Denethor had the proper perspective and knowledge. Yet Denethor was planning on denying the throne to Isildur's heir even though he knew the story and traditions of his forefathers. Twenty some generations before, the last king was slain by the Witch-king.... and some fifty generations before that Isildur cut the Ring from Sauron's finger.
Elrond had a far superior perspective of the situation and the enemy.
1. Elrond recieved the choice, of being elven or human, from Mandos himself.
2. He'd seen the Valar wage war on Melkor on behalf of the Eldar and the Edain.
3. Galadriel, his mother in-law, lived in Valinor in the light of the two trees.
4. His parents were Earendil and Elwing, legendary heroes under the very protection of the Valar.
5. His grandparents were Tuor and Idril, who knew more about the enemy's schemes and the secret plans of the Valar than anyone.
6. His great grandparents were Beren and Luthien who personally battled Sauron and even Melkor.
7. His great great grandfather Fingolfin fought Melkor in single combat.
8. His great great great grandfather travelled from Cuivienen to Valinor and back with Orome.
No one else in Middle-earth understood the situation, including the methods and plans of both the enemy and the divine allies, like Elrond did... except Gandalf (a divine ally). They were the two who set up the fellowship and the mission.
I think Eru and the Valar are intimately concerned with the lives of all the creatures of Middle-earth. But their active hands are not easily seen nor their efforts readily discerned. I think the power of Ulmo ran up the waterways of Middle-earth. Elrond's great grandfather, Turgon, followed Ulmo's leading to found a secret refuge on an old creek bed. Gondolin was the direct inspiration for Rivendell... and Ulmo may have led Elrond to it. Furthermore, Elwe (one of Elrond's great great grandfather)s had a hidden refuge in a forest, like Rivendell.
Boromir and most people did not account for the Valar nor the will of Eru when planning against the enemy. Elrond and Gandalf did.