I've never seen a good or even decent "Legion will fail" argument, ever. Bunch of dumb niggercattle who seeth at the mention of "Ave."
Unless the Courier saves his ass Caesar dies, either to the courier or of cancer, and unless there's a victory at Hoover Dam someone is going to wind up as the scapegoat, and Lucius, Lanius, and Vulpes are all going to start pointing fingers at each other... well, Lanius and Lucius will and then wonder where Vulpes ran off to.
Now, will the Legion fail instantly? No, but unlike Rome after Caesar it doesn't have nearly the same institutional inertia. Caesar was little more than a wandering tribal warlord, refusing to settle down in a city and actually build a nation until he could claim the more civilized west for himself. Most likely scenario is a triumvirate split between Lanius, Lucius, and Vulpes, paranoid about each other and the NCR crossing the Colorado to finish the job, which, assuming the NCR didn't fall apart under its own weight, is coming sooner or late simply because the Legion is a continued threat no matter what form it takes.
Image if the Legion did the reverse, from Empire to Republic, that would be very interesting.
In my hypothetical triumvirate Lucius does exactly that. I figure being Caesar's chief praetorian Caesar would like to talk to him just so he could hear his own voice while also blessing this violent brute of a man with some wisdom he so clearly lacks, and so he winds up drawing the complete opposite conclusions Caesar intended from his own misinterpreted Hegelian ideals and decides that the Legion must become more like the NCR.
"Well uh, if thesis and antithesis fight and uh a new synth-i-sis thing is formed and uh the strong are destined to prevail over the weak then uh... I guess we lost because we were weak and the NCR won because it is strong, and so if we wish to be strong we need to be more like the NCR."
Which of course is the
correct conclusion, but Caesar would be tearing his own non-existent hair out to hear Lucius put two and two together like that.