I've said before that it should've been the main focus of Genisys.

I was quite intrigued but the concept of a terminator being around for so long that it actually starts to have issues due to a lack of maintenance.

There is a few subtle moments that are never built upon, such as "pops" struggling to load the bullets when his hand malfunctions. He then checks to see whether Sarah has noticed, kyle then says "old, not obsolete" to which pops replies "not yet".

However, the film totally abandons this element of the story and just uses it as a build up to "pops" getting the upgrade.

They could've focussed more in on this element of "pops" dying and trying to keep this fact from Sarah, he then sacrifices himself to destroy the T-3000 at the end.

this further expands on the closing monologue of T2 of machines being able to learn the value of human life.