Who was she saving, though? Sheridan. Who wad Sheridan? A person she loved dearly. A character the fans were heavily invested in. Sheridan was well known and beloved.
Now, change Sheridan to a different character, a character nobody knows. A character never seen before, during, nor since. A character never on screen and never heard of before that moment. Does Delenn still willingly sacrifice for this completely unknown individual? As a fan, do you care? Is that a dramatic moment that tugs at the viewers heartstrings? No.
Now, it should. It still is heroic. It's still the ultimate sacrifice. But we're talking dramatic entertainment, not real life. It's not a compelling, heroic story. As pointed out above, viewers mourned the loss of lives on those planets Nomad sterilized, nor the crew of the Constellation. Fans aren't crying when a nameless redshirt fails to return from a mission.
Drama and heroism in fiction comes from connecting with the audience. Threaten to destroy Earth I actually destroy Vulcan and fans are involved because we feel we have skin in the game.