If you read specific parts of the novel, Zhuge Liang already foresaw Wei would eventually win because Wei had more resources (land + population) than either Shu or Wu, but not both of them combined. So Zhuge Liang’s strategy is to always attack Wei and Wei only, and hope to chip away Wei’s advantage.
There two main issues with this strategy: 1. Sima Yi was a competent enough general and administrator for Wei so that Wei could defend its lead, even though it may not be able to defeat Shu as long as Zhuge Liang was alive; 2. Wu was an unreliable ally because it feared if Shu was too successful against Wei, it would be Shu’s next target. So when Shu asked Wu to attack Wei fron the south while Shu attack from the west, Wu in general would do a half ass job, unless it expected some lasting gains.
So the inevitability isn’t something mystical or predestined, it is simple geopolitics. The novel did introduced some mystical/fictional elements, but the outcome is based on actual history.
Of course, the novel has an bias toward the old Han Dynasty, so Liu Bei was portraited as the “good guy”. But come on, even that guy in the novel was at best marginally competent as a general and at worst incompetent (with so many badass underlings he would still lose battle after battle, until Zhuge Liang came along). Incompetence is sin. (Debating who the “good guy” and “bad guy” is in the RoTK novel is pretty much a national past-time in east Asia, where the novel is very popular, especially amongst men.)