That might have seemed like folly to some people, but it was a valuable experience for others.
この場合、次のように言えますよ。
ーThat might have seemed like folly to some people, but it was a valuable experience for others.
「ある人にとっては愚行に思えるかもしれないが、他の人にとっては貴重な体験だった」
folly を使った別の例文：
ーWhat some people think is wise and wonderful, other people might view as folly.
「ある人たちにとって賢明で素晴らしいと思えることは、ほかの人にとって愚行に思えるかもしれない」
ご参考まで！
カナダ 
