You are right. When we lose a language, we also lose a lot of related histories, cultures, local information and knowledge, etc. For example, information about local species and climate patterns, historical events, and spiritual attitudes of speakers. This can also cause health problems among the original speakers, as their identity, autonomy, and spiritual sovereignty are usually highly connected to their traditional language, and the loss can lead to mental problems such as trauma and depression.
Even if the new "world language" doesn't immediately eliminate other languages, many of them are still likely to be lost. The gradual abandonment of a native language in favor of a bridge language, which allows people with different native languages to communicate, has already been a typical reason for language death in the modern period, and this can happen even faster when we suddenly get a "world language".
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