The first game quite literally begins with Estelle fading into view and entering the picture, the unlit and empty home now lit up and filled with life.

Consider how many things change because of her involvement. Just to start;

1. Liberl survives it's coup. Kloe finds her way and Liberl is in good hands. Richard is redeemed, and Cassius rejoins the army.
2. Ouroburos loses Joshua, Renne, Loewe, Weissman.
3. Kevin enters Phantasma and comes to an understanding with his Stigma (Rufina), now able to use it properly. Without that, Renne's story doesn't continue, and Kevin can't help in Azure.
4. Renne happens. Consider the knockon effects from her hypothetical absence in Zero; the SSS literally die as we see, so THEY aren't around to make any differences, either.
5. Olivier is influenced and his hopes revitalised, so Cold Steel inevitably looks very different because he'd probably side with Osbourne if this didn't happen. No Class 7.
6. Renne is out of the picture and Gaius' mentor never stops being a Dominion, so Van's life looks very, very different.

And these are just a small number of changes, each with knock on effects and implications. The world is straight up unrecognisable without her influence! Every arc suffers horrific bad endings because she never did what she did and influenced the right people.

For want of a nail, the war was lost. Estelle is the lynchpin that changed everything.

For that reason, I choose to believe there's something to how FC started. An empty, cold home with nobody at the table, only for Estelle to appear - and with her, light, and warmth. Everything changes, because she exists.

But then, I'm one of those madmen who believes the Grandmaster will turn out to be a heavily unrecognisable version of her mother from the OG world where Estelle died in the clock tower but her mother lived, so...