So, agnosticism also refrains from claiming this. The open-ended view of the single life which can be perceived prevents even this assumption. The result is "work with what you have", and it then becomes a choice to decide what one's concern will be.
If dukkha becomes one's concern, then the Dhamma can be practiced with increasing benefit while holding Buddhist cosmologies with agnostic hands.
I used to think so too, but increasingly I'm convinced that it's better to have a firm view one way or the other. The reason has to do with the nature of the Buddhist path and what it requires of practitioners.
The path is demanding and requires great sacrifices. One is urged to abandon most the things which make the world tolerable -- intimate relationships, family life, art and music, culinary pleasures, a rewarding career or vocation, and so on. If this brief life is all we have, it is pointless to give up these things. Not just pointless, but a sad waste of one's time here on earth.
On the other hand, if beings really are caught in an endless cycle of (mostly miserable) lives, with all pleasures becoming pain, and all pursuits turning out to be empty, then renunciation makes sense. We have good reason to struggle hard and endure constraints/discomforts in order to break free of the cycle. The Buddha's path requires a full commitment ("practice like your hair is on fire") which is justified by the urgency of escaping the samsaric prison. But if we have only one life, there is no reason for urgency, as we will all achieve cessation at the moment of death.
Thus, it seems to me that agnosticism ultimately won't do. At some point a decision must be made because so much hinges on it. For non-believers like me, there is still benefit in Buddhist teachings and practices, but only to a point -- perhaps with the sort of non-religious approach advocated by Sam Harris