Is it necessary to strive to improve the present, despite it being a fleeting world, and for fear of forgetting the Hereafter?

Yes, we strive to reform reality even if others strive to corrupt it, because striving for reform is an act of worship and jihad in the path of Allah, and one of the reasons for salvation on the Day of Judgment, as it necessitates the establishment of proof against the corruptors. Allah knew of the corruption of people, yet He commanded reform and sent messengers. And from His wisdom, glorified be He, is the struggle between truth and falsehood, to distinguish the truthful from the liar. Undertaking reform despite the transience of this world is an obedience to Allah's command to build it up, just as the greatest reform is inviting people to monotheism and obedience to Allah. And the ultimate outcome is for the righteous, for no matter how much corruption rises, Allah will enable the reformers to repel it and restore the Ummah to ascendancy.