Spike has had enough of Twilight blaming herself for G5's future. And tells her to snap out it.

Spike: Listen Twi, the future you saw, is not your fault! Our past is described as "Ancient" Equestria, meaning hundreds to thousands of years later! What ever happened to make Equestria become segregated had nothing to do with you! What your doing DOES matter! When we were in the human world, I actually did some reading of human history..since I was a dog, I had very little to do so…anyway, I learned of empires that are very similar to Equestria, but they fell despite lasting for centuries, it had nothing to do with the founders, it had to do with the successors.

Twilight: But I fa..

Spike: STOP! You did not fail! Look at what you would eventually end up doing, you proved it was possible to not only preserve friendship between the Ponies, but with all creatures, look at how much you accomplished despite all the odds! And look at the future again, look at the Mane 5! The teachings you made, mattered! If you did nothing, if Sunny did not have your legend, she would have been just another racist.