Depends on if the corps have been sitting on some phased plasma weapons they haven't been telling anyone about.
On the other hand, if Skynet is purely an aggregate software intellect (think T3), a pure AI, like the things living beyond the black wall, then be damn glad there is a wall because you don't want something like Skynet deciding to reprogram humanity through cyberware or just deciding to system reset, synapse burnout, or cyberpsycho the bulk of humanity. Hell, it'd be trivial for it to make us all sit docilely in a happy-fun braindance while its murder machines "sterilized" the planet.
There's a reason Netwatch pisses itself at the thought of anything like Alt coming back across, and Doctor Paradox tells people to stay the hell off the Net (well besides him wanting people to unplug in general).
You really want something like Alt going all Skynet or Shodan on us? Just imagine all those drones Militech and Arasaka love to use simultaneously deciding that your city needs to die, or a Militech satellite decides to drop a dozen or so "rods from God" on your city.
Also, terminators are made of a "hyperalloy combat chassis (coltan steel)" that would hypothetically be all but indestructible to anything but anti-tank and anti-material level weapons. So good luck taking one down with your smartgun. MAYBE your tech rifle will punch a hole in it, you'd better hope. The best you can hope for is that you'll be able to fight back marginally better than John Connor and his merry band of survivors, assuming Skynet doesn't make you suicide yourself or simply switch you off.
Yeah, every scenario of that sort is an extinction level event, which is why the Voodoo Boys are trying so hard to find an exit.