Not true, the Toyotomi were doomed to fail even before the battle of Sekigahara. Hideyoshi was a known 🍆, he imposed ridiculous rules, his family was mess, he crucified christians (which Ishida did to Akechi’s daughter and destroyed his reputation) and let’s not forget those absolutely disastrous campaigns in Korea. The Tokugawa were gonna take over regardless
Would be quite a challenge for Mitsunari and the Toyotomi to stabilize the land in a scenario where they managed to not only win at Sekigahara, but also effectively eliminate Ieyasu and most of the Tokugawa supportes. Unlike the Eastern Army which were all united in support of the Tokugawa, the Western Army had quite a handful of daimyo who joined not necessarily because they supported the Toyotomi's rule, but because they didn't want Ieyasu to take over and saw Mitsunari as the lesser of the two evils. Mitsunari's victory would definitely have helped the Toyotomi with remaining the big authority in the land and the ones who were loyal to him would be given more powerful positions, but the Sengoku Jidai would probably go on for a couple more years because of those daimyo whose loyalty was always into question inevitably going back to fighting for power until they get subdued. Plus, Mitsunari would have to make sure not to repeat Hideyoshi's mistakes that almost led to his clan being ruined.
