Clip art images like to highlight the keystone as the one stone that finally makes an arch. This suggests that it's the leader. It's the bipartisan connector that brings an arch to life. It's the one that finally connects both sides.
It's not entirely true.
For example, the left side of the arch would not be connected to the right were it not for the lowly second stone on the lower right side.
In a stone arch every stone counts.
Mathematically the stones in arches of shapes other than semi-circular arches all experience different shear forces. Regardless, without every stone doing its part, big or small, the arch fails to exist.
The COVID19 curve is same same but different.
A curve is not an arch, but the pressure on each part of the curve is a result of the shear forces between coordinates. The line moves up and down relative to the force of the previous coordinate and the space between the bases. Additionally, the line moves up and down the same way on both ends.
This is like building a small stone arch in a back yard stream. We can quickly make two tall piles of rocks close together and then tip them towards a perfectly weighted keystone and call it an arch. Gravity and friction--without a supporting structure--allow for this.
If we were to attempt to build a long, low and flat arch without support, the forces of nature would get the best of us.
Long flat arches are possible but require removable scaffolding and more careful calculations of friction from stone to stone.