In terms of influence, the series starts by being influenced solely by the 70s era of mecha, specifically, the Super Robot era defined by series like Getter and Giant Robo, and with each arc adding a new era. The 80s, defined by real robot shows like Gundam and transforming mecha shows like Transformers and Macross, is added in the second arc. The 90s, defined by the darker takes on the more optimistic super robot and dramatic real robot formulas, most notably Neon Genesis Evangelion, is added in the thrid arc and coincides with the introduction of the Anti-Spirals. The modern era of mecha, which is partly a revival of all the previous mecha eras, is represented last. Rather than shifting influence, it really just adds, so that in the end we are left with mecha genres stacked atop each other, with the mecha representing the super robots of old facing off against the faceless unknown evils of the Eva-era mecha.
Super Robots represent hope. Eva-era Mecha represent despair. Spiral Vs. Anti-Spiral.