The point is that they said clearly that these beliefs are from Sinai / divine inspiration, and they were certain that the Sages knew this as a fact. And, as you said, you too would believe it in such a case. So why is it that when Maimonides claims something is from Sinai, you decide not to believe him, but if the Sages had said it you would believe them?
And if you think the Sages would be reliable on this matter, but Maimonides probably had enough reasons to infer it, then I’m asking: what do you think those reasons were?

First, I don’t recall that Maimonides said this.
Second, even if he did say it, it doesn’t appear that he has a Talmudic source, and this is probably his own interpretive reasoning that it is presumably from Sinai. More than once, you find medieval authorities concluding on the basis of reasoning that something is probably from Sinai even when the Talmud doesn’t say so. The Sages sometimes base these principles on very unconvincing homiletic readings (“You will lie with your fathers and rise”).
Maimonides certainly took philosophical and scientific truths as binding truth (see Laws of the Foundations of the Torah), so he is certainly suspect in this regard. Am I also supposed to accept the existence of the spheres and the opening chapters of Laws of the Foundations of the Torah because that too is from Sinai? This also raises the possibility that the Sages did the same thing (“our water,” “the sphere stands still and the constellations revolve”)—taking something to be true based on reasoning, or because that is how the world in their time understood it.
Precisely because there is reason to assume these principles (it solves problems of divine justice), that only further strengthens the possibility that the Sages created this through logic.

What does the Rabbi think?
Is there probably reward and punishment? Is there a World to Come?
What seems plausible to you?

Reason suggests that if we have a soul, it does not perish with the body. And the logic of divine justice also suggests that there ought to be some stage at which compensation is made for the injustice in this world. But both of these are just my gut intuitions; everything else is detail, and I really have no idea where it was drawn from.