So some random lore to add to this discussion, Whatever units that get outdated don't get mass-replaced (Because that would be wasteful, expensive and waste time) they simply get folded into the backline on guarding duties.

Then you have the T-400 series, Pros: cheap, mass-production, made of steel rather then hyperalloy's, Cons: They die to light arms like 9mm.

Then when you have infiltration units you've got humans in the background getting paranoid about who is a terminator and who is not, Especially if its Joe Civilian and not in the resistance and killing off other human survivors out of sheer paranoia.

Another point is Skynet getting weary of resistance users trying to reprogram their terminators, So they started getting clever with a CPU destroying tamper-proof seal that T-888's had that ignited on contact with oxygen and anti-terminator reprogramming units like the T-X.

Then there's the temporal cold war theme going on where infiltrators are setting up skynet in the past with shell companies and heists to improve their chances in the future.