SkyNet isn't the Borg. It couldn't reconstitute itself from only a Terminator or two. Its mainframe had been destroyed. There's no recovery from that.
And besides, since the Resistance is qualified to operate the time machine and knew exactly where to send Kyle, it stands to reason that they would have detected it if SkyNet had sent a terminator into the *future* - they'd juat have to wait and then slag the thing as soon as it arrived.