A win against Brighton would have improved Arsenal's title fight with Manchester City, but Arsenal's 3-0 defeat against Brighton meant that Manchester City went well ahead of Arsenal on points.
Arsenal are 4 points behind Manchester City after playing one more match. If they had won against Brighton, Arsenal would have had 84 points and Manchester City would have had 85 points, that is, the two teams would have been equally ahead in the title fight, but it did not happen.
Manchester City may be going for the league title along with the Champions League title this season.
At some point, Arsenal's lack of consistency caused them to lose games and points, which had an impact on their hopes of winning the Premier League. They eventually lost to Brighton, a game that would have easily put them back in contention to win the trophy after Manchester City had flipped them on the table, but because they were already 4 points behind City, winning any game wouldn't have any bearing on their chances of winning the league. They need to improve their preparation for the following season because Manchester City might end up taking home three trophies after all, and they did work hard for it. They should learn to be more strategic the next time dreams are already lost.