>inb4 hurr teaching people to build things from scratch instead of scavenging
People in NCR territory are already building things from scratch, the Gun Runners have been doing it since Fallout 1. The Legion is behind on this, which is why they're trying to do business with the Silver Rush.
>inb4 durr muh synthesis
The only synthesis that will happen once the Legion takes the Mojave is that they'll overextend like the NCR did, because the actual NCR isn't in the Mojave (which they don't own). The Legion has to keep conquering to avoid becoming stagnant, but won't be able to expand any further once they take Vegas. The Legion is even more fucked in the long term than everyone else.