hashkafah philosophy - Why is Moshiach a “fundamental belief” (iqar)?

In what sense is belief in the eventual coming of Moshiach fundamental to Judaism? Rambam famously includes it as the twelfth in his list of iqarim, but why?

Considering the other iqarim, Judaism would be very different without them—non-existent or nonsensical without some. But it seems to me that if we had been promised something else in place of a restoration of the Davidic monarchy (a generic “Messianic Age”, or a Torah republic, etc.) there wouldn’t be much difference in Jewish practice or thought.

Note: I’m not asking whether belief in Moshiach is required; I’m assuming that since Moshiach is prophesied, belief in the eventual redemption and restoration of the Davidic monarchy is a necessary consequence of belief in nevu’ah. I’m also not asking why a melekh hamoshiach is needed (rather than the alternatives I mentioned above). I’m only asking why (or, in what sense) this belief is part of Judaism’s foundations rather than being one of its consequences.

Answer

It is part of the fundamental principle of yichud Hashem - that not only is there only one God, but God alone has full control of the the world and His will reigns supreme. The Ramchal starts off klach pitchei chachma discussing this. (See also Daat Tevunot).

There he explains that everything in the world, (even what a human being does with free will such as sinning) must be authorized by God. Even when a person commits a sin, it must first be authorized by Him and He considers how it will fit in His divine plan of total rectification in the end. This rectification will be accomplished through either repentance or punishment.

Without this fundamental belief, the Ramchal explains, one can assert that the Jews sinned against God and "ruined" His plan, and are therefore eternally damned (as some Christians believe).

Hence, the belief in Messiah is the belief that history is not a chain of events going nowhere, but there is a plan for creation and that God has exclusive control of that plan and history is inevitably moving towards this goal.

(this last statement by an audio from Rabbi Uziel Milevsky zt'l)