@DoctorWTF
Yeah when you end a series with “and all the races came together and were happy” starting the ‘sequel’ with “so eventually a race war broke out” is not a good sign.
Nothing lasts forever. It is also not surprising that conflicts should break out between different peoples or “races” sooner or later, as times of crisis can trigger resource scarcity and instability which will thus lead to quarrels and then open hostility. By the time the G4 ended (Season 10 / Last Problem), Twilight Sparkle had expanded Equestria across all known boundaries, and may eventually have become an empire based on friendship - which foresaw the element of stability. What will happen if this disappears, if Twilight either lost the motive to rule or retire without anyone being able to take over after her? If there is one problem that cannot be cured among the ponies, it is that they tend to have a lack of control over their emotions and can have serious communication problems when they need to vent what is bothering them. The alicorn committee may have concluded that this had to be brought under control, thus introducing a strict regime in place to preserve what Twilight had left behind, but did not pursue her ideology which was essential to the common society.
So MC who had obviously been inspired by the legends of the Mane 6 which may be a thousand years old in his time, will do something because he felt insecure about the alicorns and their way of governing, if they were to have an autocratic regime which must have tight control over Equestria for the sake of peace. He will thus bring forth friendship again, and give the people new hope where they can learn how to live together again. And it is possible that the movie will show the villains who had spread the corruption, before the elements are used.
Thus, the G5 can start in earnest.