At a recent showing of 28 Days Later, a vast majority of the audience got up and left the theater well before the closing credits stopped rolling. None of those people, in other words, bothered to sit through the downbeat ''alternate'' ending that was tagged onto Danny Boyle's zombie thriller starting July 25. I can't say their lives will be any less rich for it. The revamped 28 Days Later may, in essence, have been a marketing ploy, but I still hoped that Boyle, by restoring the gloomier finale that had divided test audiences, would cast a no-future spell. Alas, the new version ends not with an apocalyptic bang but with a whimper of soggy pessimism. (Fear not: I won't spoil the anticlimax.) Perhaps now alternate endings will stay put on DVD, where they belong.