AN10.95 is the answer.
In the same way, it’s not the Realized One’s concern whether the whole world is saved by this, or half, or a third. But the Realized One knows that whoever is saved from the world—whether in the past, the future, or the present—all have given up the five hindrances, corruptions of the heart that weaken wisdom. They have firmly established their mind in the four kinds of mindfulness meditation. And they have truly developed the seven awakening factors. That’s how they’re saved from the world, in the past, future, or present. Uttiya, you were just asking the Buddha the same question as before in a different way. That’s why he didn’t answer.”
It’s left undeclared.
If the Buddha were to say that everyone will eventually be freed and attain to nibbana, marking the end of sentient beings in samsara via no more rebirth, then it doesn’t matter if the physical universe goes on or not, no more consciousness to observe it. That’s the end of samsara.
But the practical aspect is that if he would had declared that, some people would be lazy. Since liberation is guaranteed, they rather not work now and get paid later, eventually.
If Buddha were to say that samsara has no end, then some people would think that “oh, I must be one of those people who can never attain to enlightenment. What’s the point of practising hard?” Then they don’t put in the work and become a self fulfilling prophecy.
The reply in the sutta makes sense to spur people to practice.