The Legion is a totalitarian slaver organization obviously. On a more important note the Mojave Campaign is the end of the line for them.

Even if they take the Mojave
A, They'll lose the Mojave to the tunnelers
B. Legate admits in his last speech to the courier that yes the NCR will continue to send harassment against them hurting stability
C. The divide creates a natural barrier against them taking California, ending their easy targets for military advancement

All of those three things present existential problems for the Legion but unforturnely for them chances are they don't take the Mojave

In 3 out of 4 game endings Oliver wins the Second Battle and arrives at the fort to kill Legate. Even in the most neutral game ending (No robotic army, no outside faction help, independent courier) Legate still loses the Second Battle. Which means most of the Legion's top troops are dead or going to be dead soon, they've spent 3 years worth of resources on a failed campaign and once Caesar dies so to does his cult of personality.

The Legion wouldn't last 5 years past 2281.