The original Superman franchise suffered from some "death of the franchise" moments. Those can be largely attributed to bad management. Richard Donner was kicked out half way into Superman II, and Dick Lester brought in. Lester would take the helm for 3 and 4 as well. Lester introduced humor, which was out of place in the series, and which could be said to have attributed to 3 and 4 being so poor. But I don't think he's all to blame, and I'm not sure exactly who is at fault. But 3 and 4 were lackluster, and killed the series. And the spinoff "Supergirl" was a bomb.
So the question is, what if such a fate did not befall the Superman film franchise, and the 3rd and 4th installments were good? Could a 5th have been made soon after, bearing in mind Christopher Reeves paralysis would also likely be butterflied?