Why is the fall of Western Rome significant?

The collapse of the Western Half of the Roman Empire serves as one of the critical moments in European History, marking the divide between Classical and Medieval History. The Roman Empire created one of the great empires of world history, and its collapse resulted in political fragmentation throughout Western Europe, the implications of which would reverberate for centuries afterwards. At the same time, we must also recognize the great amount of violence which was present within the Early Middle Ages, which required that new political structures emerge to restore stability and fill the power vacuum that had emerged with the empire's collapse.
From here, we find the core themes of the Middle Ages taking shape. While Roman political authority might have collapsed, the Roman Church remained. Thus it served as the primary source of large-scale unity as well as the most powerful institution in this post-Roman world. For this same reason, the Church also emerged as the center of intellectual activity during the Middle Ages. Meanwhile, the institutions of Feudalism took shape, reflecting the political fragmentation that shaped it. Even so, the memory of Rome would linger through the Middle Ages and beyond, exerting an influence even centuries afterward.

The fall of the Roman Empire marked the beginning of the Middle Ages: a period of time known for widespread poverty, religious tyranny, and intellectual and technological stagnation that lasted for a thousand years.
For about five hundred years (27 BCE through 476 CE), Rome was the most powerful empire in the world. At its height, Rome held sway over the Mediterranean region and most of Europe—over two million square miles. The Roman rulers had to consider the needs of citizens from many different cultures. The system of government was designed to promote efficiency and growth, which is why it worked so well for so long.
This enormous territory was difficult to govern from one central location though. The newly founded Christian religion gained popularity in the third century CE, creating a divisive and volatile religious rift. In 313 CE, Emperor Constantine, a Christian convert, declared Christianity to be the official religion of Rome. He moved the capital city eastward to Constantinople, dividing Rome into two separate empires and disrupting the systems of government and social structure that had held the empire together.
Eventually internal division, social decay, and external pressures created a situation in which Rome could not hold on to its power. Western Rome faltered and eventually fell to German invaders in 476 CE. With the Christian Church in power throughout Western Europe, technological and social innovations came to a halt and the "Dark Ages" began.
https://www.ancient.eu/article/835/fall-of-the-western-roman-empire/