But what exactly did Shimon HaTzaddik see in the Kodesh Hakodashim that convinced him he would perish?
Rav Soloveitchik explained that each year when Shimon HaTzaddik entered the Kodesh Hakodashim he experienced a feeling of optimism. But that year he experienced a feeling of despair regarding the continued existence of the Jewish people due to their religious and spiritual decline. When he experienced that feeling, Rav Soloveitchik explained, he knew he could no longer serve as a leader of the Jewish people.
In other words, when we lose hope, we perish. We must never lose hope.