When Zamenhof launched the language in 1887 many people were convinced that it would soon fragment into mutually incomprehensible dialects because Esperanto was not a national language. This never happened in spite of Esperantists being notoriously individualistic! Why this did not happen is a subject for serious scientific research; but for some reason linguistics has managed to ignore the existence of Esperanto. (Perhaps specialists find its existence embarrassing because of a hang-up concerning planned languages!)
I think that initially Esperanto maintained its unity because of strong ideology; in other words, disunity would lead to lack of comprehension, and that had to be avoided at all costs.
The ideological factor is still present and is of importance. But perhaps a more important factor was the rapidly achieved stability which made it impossible to effect change. Try changing your own language — even the must absurd elements — and you will get nowhere. This is because of inertia in the technical sense. In the case of Esperanto the key realisation was its being a language and not a project.
Yes, Esperanto is all sorts of things; but most fundamentally it is a living language. It is incorrect to allude to the languages of the world and then append Esperanto. You may love it or hate it; but Esperanto is a human creation; and all languages are human creations: Esperanto was created, and the fact of its logical structure does not mean that it is not a human language — unless you wish to claim that a language must be unstructured, illogical, and an exercise in the absurd!
The first Esperanto writings of 115 years ago can be perfectly easily understood today. The differences are not semantically important.
Has Esperanto grown during this period? Yes it has. Its vocabulary has increased because new concepts have come into being. It is not a language which is sprinkled with incongruous borrowings because certain spheres have been abandoned or neglected. I shall not name languages which belong to this category, but examples are well known. International terminology belongs to Esperanto even before the words are used by Esperanto-speakers for the first time. Terminology is phoneticized and made to conform to Esperanto structures. This is a key principle which permits the language to refresh itself. On occasion terminological variants may appear; the Esperanto community decides which variant will become normative.
Another factor which is important for Esperanto is the right of all speakers to manipulate the language. Anything is permitted which does not violate the fundamentals of Esperanto structure. The results can be surprising; but comprehension is not normally a problem. It is unusual for the speakers of a language to be allowed poetic freedom! Paradoxically this freedom strengthens the foundations of the language. Esperanto is not a fossilized language! Zamenhof supported this principle, but was deliberately conservative during the early days. However, his writings pointed towards the future!
Esperanto has kept its unity. This is not an assertion: it is an historical fact. The present communication situation is replete with possibilities for using Esperanto. The Internet has a pivotal role. There are more opportunities for using the language than ever before: conferences and meetings abound. There is a vigorous original and translated literature. Travel was unusual during the lifetime of Zamenhof; it is now part of everyday life. All evidence points towards the essential stability of the language and also to its capability to remain pertinent, contemporary, and semantically sophisticated.
David W. Bisset