How do you rationalize audio upgrades?

It's pretty simple for me...can I afford it. Once you get to a certain level it cost a lot of money to get relatively minor improvements. 

Desire. It’s a thing to itself

+1 @dramatictenor

Assuming all other factors have been examined, and there is a 'real' difference,

At 71, I am very realistic regarding 'will I use it? benefit from it? resale value?

Like you, I look at the extra money over time to help decide

cameras: I look at the extra cost over 3 years for a camera, because technology, features are still progressing. I get a 3 year accidental warranty, expecting to get a new model when that is done. I also wait, stay back a generation, to save cost and let others prove reliability 

audio, a much longer, or nearly forever decision, technology changes, if any, even if real, don't often pass a blind listening test,

it will be for a long time, so I first clarify required specs and features, then exhaust myself in research, ... I don't want to learn about something I didn't know about a week after I made a decision. 

Also, is something better because of something else, or itself. Is something great dismissed because it is compromised by something else (someone messed up a camera's settings, someone left the loudness circuit on, ... so many factors ...

yes, over time.

I think it is also fair to say that in many cases, tastes, wants and desires change over time...and if/when this happens, your current audio system may no longer provide the enjoyment that you expect which then requires some "tweaking".