I suspect my more “senior” colleague on my team consistently thwarts my ideas and continues to make bad programming decisions because no one else wants to deal with the code we own and he’s just trying to have job security by making it so that he’s the only one who understands this bowl of spaghetti.

Eurgh, been there. I feel that one :(

I don't think most programmers are inclined to that kind of sly career thinking.

It's more likely that he's got nostalgic feelings for his spaghetti code or some very niche specific reason for preferring it.

I can relate: I built libs years ago for stuff that later became part of some standard lib. When someone wanted to replace my lib with the standard I was at first offended and found some details that my lib did a bit better - without seeing the big picture.

But after some time I came to realise the standard was superior.