[quote=@Andronicus23]
Even if the Legion lost at Hoover Dam, its entirely possible they still survived as a faction. A strong enough leader (aka whoever takes them over) could step into Sallow's shoes, claim descent from Mars as the next Caesar, and reorganize the Legion. 
[/quote]

I have to disagree. it's repeatedly pointed out in New vegas that the Legion is likely bound to fail, and with Sallow bound to die eventually, I don't see it not imploding, whether violently due to his death or more slowly due to simply not being a very sustainable society