Why stopping a boarding party matters before it reaches the deck

Would a modern boarding action necessarily be a disaster for the attacker?

Not necessarily; the critical defence is keeping the attacker off the ship, because containment becomes much harder after boarding succeeds.

Modern boarding is not treated as automatically impossible. Warships offer fewer access points than older vessels, but that mainly helps while the attacker is still outside. Once a boarding party establishes itself on deck, the defender faces a more complex internal security problem. The exchange shifts attention from cinematic close combat to the layered perimeter that is supposed to prevent contact in the first place.