[Throwaway90283]

Upgrading their service is expensive. T-Mobile upgrades, then Sprint has to upgrade to compete, then T-Mobile has to upgrade again, then Sprint...

Instead what's going to happen, T-Mobile and Sprint both agree to offer the same performance, or focus on different regions.

It's like selling slices of pizza on the street. If you're smart, you and your competitor agree to sell at $5/slice. If you drop to $4, sure you'll get more customers for a short period of time, but then your competitor drops to $4, and now you both just cut 20% off your revenue for no reason.

[striking]

Further reading: https://en.wikipedia.org/wiki/Nash_equilibrium

[javert]

Technically, the pizza idea won't work, because more people will enter the market and compete on price. Doesn't apply for telecom since it's all monopolized and new competitors can't enter.