So are the words. All of them have 'a service life' and then get replaced with more convenient and useful ones in new circumstances and new reality.
The 'long-livers' among the words are conjunctions, prepositions, suffixes, pronouns and articles. They are the ones which are most in demand, and therefore are present in any texts. They inevitably become the most convenient to use, since over the centuries they have managed to be 'polished' in millions of lips - like pebbles polished by waves and sand. These words are always the shortest, easy to pronounce. And the brevity and simplicity of a word are the apparent signs of its antiquity.
Valeriy D. Osipov, PhD