English Translation
Everyone cheers for the winning team.
Hhhmm… I feel this should be 応援します, no?
Everyone cheers for the winning team.
Hhhmm… I feel this should be 応援します, no?
優勝チームにみんなで応援します (your proposed sentence) is grammatically broken.
優勝チームをみんなで応援します is grammatically correct, but it means “We all root for the champion team of the past series/tournament.” In other words, the team became a champion last year, so everybody roots for the team as a defending champion this year. Now is in the midst of the series/season. And everybody doesn’t have a favorite team and is rooting only because the team is a defending champion (maybe for sports betting or something??)
優勝チームにみんなで乾杯します means “We are going to make a toast to the champion team of this year/season.” Unlike the second sentence, this third sentence (original Japanese sentence) suggests that the season/series is just over this year. And people are now preparing for a celebration party.
The English translation “winning team” is incorrect or at least misleading. 優勝チーム refers specifically to the champion of a series/season/tournament, which consists of multiple games/matches. On the other hand, a “winning team” can refer to a winner of a single game/match. Therefore, a “winning team” can even rank at the bottom of standings. A “winning team” should be 勝利チーム in Japanese.