A monopoly becomes illegal when it negatively impacts consumers.

If a company is able to lower costs to better compete with another company taking market share, wouldn't that imply that:

1. They had a defacto monopoly in the sector that allowed them to price above the fair market value.

2. They harmed consumers by pricing above fair market value.

No, it simply means that the market conditions changed. The price could very well have been a fair market value before, and still is a fair market value after; and the delta of these prices reflects the impact of the new conditions.