I think the main points are the following:
1. We must do all we can to inform others of Buddha’s teachings. It is out of our hands how many would be interested.
2. However, the Buddha discouraged anyone from forcefully teaching Dhamma to others or engaging in “debates” that may escalate and become unproductive.
Regarding who is qualified to teach:
1. The “Ministry of the Buddha” or the “Buddha Sāsana” with the unique message of the Buddha will last only as long as there are Noble Persons (those who have attained magga phala, i.e., Sotapanna through Arahant) stages.
– They have the special name “kalyāṇa mittā” of a “Noble friend.”
2. Only a Buddha or a true disciple of the Buddha (who has grasped the teachings of the Buddha) can explain those teachings.
– Once there is no longer any such kalyāṇa mittā on the Earth, Buddha Sāsana will disappear. Full copies of the Tipitaka may be there, but there will be no one to explain their contents. This is also why “word-by-word” translations are useless: “Elephant in the Room 1 – Direct Translation of the Tipiṭaka“
– Then, the world will have to wait for the next Buddha, Buddha Maitreya, to be born to attain magga phala (Nibbana.) That is supposed to happen in a billion years or so.
– The current Buddha Sāsana of Buddha Gotama is expected to disappear within the next 2000 years.
3. That is why the Buddha told Ven. Ananda (in the Upaḍḍha Sutta (SN 45.2)) that the continuation of his Buddha Sāsana depends TOTALLY on kalyāṇa mittā.