The Empire is on a decline... But also they have united after a thousand years of civil war, have thriving academic institutions, rapid advances in technology, and for the first time, a prosperous middle class.
The Dwarves are on a decline... But the holds have reconnected after thousands of years of isolation, runelords are uncovering ancient treasures left and right, the engineers guild is the most technologically advanced organisation in the setting, and their infamous wealth is being turned towards reconquest rather than survival.
The elves are on the decline... But they have forged mutually beneficial trade treaties with the human realms, dominated the great ocean despite a dozen hostile factions, and are lead by the greatest heroes of any age.
Chaos is on the ascent... But their champions are constant fighting each other, their schemes are short sighted, the gods are constantly sabotaging each other, and it's all doomed to failure due to the inherently self destructive nature of those who chose Chaos over literally any of the other alternatives.
What Warhammer says and what Warhammer does are often two different things.