Axel,
I too am for all that you say there.
Yet the fact remains that all those who attained during the Buddha’s time, and even afterwards, did so on listening to a desana. The uncertainty that we face is because at that time the Tipitaka had not yet been committed to writing, to say nothing of printing. There were no options to compound the issue. No wonder the suttas are silent on the matter – the question simply could not arise.
But what about previous Buddha sasana (pl.)? It is altogether possible that writing, printing and other forms of communication had been developed in this and previous kalpas during a Buddha sasana. In such a case, a jati-Sotapanna who has developed the power to see into that past life could help!