Once you announce the imminent introduction of a new generation, you risk losing sales of the current generation, as prospective buyers give up on the current model, thinking that with the new model coming, they will just wait for the new car.
This has happened to the Camry. The Camry was once the best-selling car in the USA, maintaining the status for years on end, except for one year, when a new generation was imminent. Buyers decided to wait for the new model, giving up on the then current model. Learning from that, in the years when a new generation of the Camry was pending, Toyota would push the outgoing model, all in order to maintain best-selling sales of the Camry.
But if the wait is too long for the new generation, you risk buyers losing interest in the current generation, then also losing interest in the upcoming generation because they give up waiting so long for the new car.