Throughout the history of Buddhism there have been controversies about the ideas of irreversibility and of determinism.These are to do with the inevitability (or not) of future arrival at nirvana, either for individuals or for all beings.
Thus, in the Mahayana, in many scriptures, Buddhas give predictions of enlightenment to individuals or to whole groups. These are thereby singled out for their faith, having reached a degree such as will inevitably put them on an upward learning curve from which there is no return. In the Pali canon too there is the well-established typology of four grades, namely, arhat, non-returner, once returner, and stream entrant. These are all categories of persons for whom enlightenment is either already attained or inevitably assured. In Pureland, there is the idea that those who have faith are assured of entry into Sukhavati and that the conditions in Sukhavati are such that enlightenment is ultimately assured. Thus, in a sense, those who enter Sukhavati are "non-returners" unless they have made bodhisattva vows in which case they abjure entry into nirvana until all sentient beings can do likewise.
The fact that some are irreversible implies that others are not. Thus there is no inevitability about all beings reaching nirvana ever. At the same time, many Buddhists in history have believed that it was inevitable that all would get there eventually. Sometimes, of course, it is rather difficult to tell where hope stops and belief starts since both deal with matters that cannot be known for sure.
Now, in Buddhism belief is less vitally important than it is in some other religions. It is more the sentiment of faith that matters rather than the precise definitions of belief in this or that doctrine. Since one of the primary beliefs is that we are all deluded, one can hardly expect the ordinary practitioner to understand everything perfectly. These various ideas can be seen as the way that different groups of practitioners or different teachers have given expression to their faith in different times and circumstances. So I will attempt here to give my own take on the matter, but this does not mean that what follows is holy writ. Each person finds his or her way.