Upgrades got you broke?

I started reading Mastering Mountain Bike Skills by Brian Lopes and Lee McCormack. In the first few pages it talks about picking the right bike. I was surprised to see that they recommend taking it easy on the upgrades and just go ride your bike the way it is.

But they do give a list of things to consider when upgrading.

1.Saddle “its hard to have fun sitting on a plastic anvil.�?
2.Stem “they should fit your body and riding style.�?
3.Tires “Choose ones that match your riding conditions.�?

They basically said for you to ride your bike until something breaks or wears out.