Pre-viability and post-viability feel radically different.

For example, in a product development context, pre-viability might mean pre-PMF.

Pre-viability is like pushing a rock up a hill.

All of your effort has to go into motive force to push it up the hill.

If you ever get distracted the rock rolls back down the hill.

Post-viability is like skiing along behind the rock as it tumbles downhill.

The rock moves on its own, and you mainly need to help steer its path.

The rolling rock has momentum.

Everyone participating can see the direction it wants to travel and is traveling on its own, and can help make that happen.

The momentum is a natural coordination mechanism.

The difficulty of pushing the rock up the hill is both the weight of the rock and how many rocks it is.

A single rock is possible–with patient effort–to push up the hill if you're strong enough.

But when there are multiple rocks, they constantly roll off to the side and roll down the hill.

Keeping them together is just as hard as pushing them up the hill, if not harder.

In the limit, the hardest would be pushing water up a hill; it's impossible, it just flows around you and down the hill.