Always,
1) It’s not that we can’t follow Rishonim unless acharonim go the same way. It is that we don’t change the accepted psak based on the writings of a Rishon that were only re-discovered after the psak/hanhagah was accepted.
2) We typically follow rov Rishonim in halacha but not all Rishonim are equal. The Shulchan Aruch went with the Rif, the Rambam, and the Rosh and followed the rov of those three. The Rama held that Tosfos, and others, should be included too. But everyone agrees that the “shita lo noda l’mi” doesn’t impact halacha.
3) While we follow rov Rishonim, m’ikar hadin. We can be meikel like minority opinions or be machmir like minority opinions in certain situations.
And of course, there are plenty machlokesim among acharonim.