In universe, why wouldn't Skynet use biologic/nanotec infiltrators (instead of Schwarzenegger)/subtle WMD to abolish mankind? Surely it would mean they'd win before we knew we were at war.
Two main reasons:
1) Skynet needs human beings to survive until Judgment Day. If it sends an engineered virus back to the 1980s and wipes out humanity, at that point Skynet itself has not been built, so Skynet will never exist.
2) Sending one or more Terminators back in time was a desperate, last-ditch move by Skynet. It didn't have much time to prepare.
The second point is strongly implied by Reese's dialogue with Silberman:
SILBERMAN: Why this elaborate scheme with the Terminator?
REESE: It had no choice. Their defense grid was smashed. We'd won. Taking out Connor then would make no difference. Skynet had to wipe out his entire existence!
SILBERMAN: Is that when you captured the lab complex and found the, uh, what was it called... the time displacement equipment?
REESE: That's right. The Terminator had already gone through. Connor sent me to intercept him and they blew the whole place.
SILBERMAN: Well, how are you supposed to get back?
REESE: I can't. Nobody goes home. Nobody else comes through. It's just him - and me.