You’re right ; ‘the most influential’ and ‘the most fashionable’ both mean ‘people’, and this is understood. Both are noun phrases, consisting of a noun ‘people’ [understood, though not stated], an adjective [influential, fashionable] and an adverb [most]. ‘The rich’ is similar, but in this case we don’t think of ‘people’ as a missing word because the phrase ‘the rich’ has become well established.