The 1K has demonstrated continuing current value to UA.
"Lifetime spend"? Funny. UA is looking for current and continuing value. If you die after flying 5 million miles with United, they're not going to put a plaque on a bulkhead in your honor.
A one-off full fare passenger is just current value. More valuable than a (most) Basic Economy customer(s), but less perceived value than a customer who is demonstrating current and continuing value.

You are deceiving yourself as to how UA values you at any point in time. It really is variable. If you've qualified for Silver, you will be treated like a Silver, not like you were 1K at one time.

Think of it this way - United values it's customers on "What are you doing for United nowadays?", not "What are you doing for United today" or " What have you done for United in the past?". 1Ks and GSs have the answers UA likes for the first question.

When I quit flying like I do, I'll sink to Gold courtesy of MM. I won't be wondering why I'm not being treated like the 1K I was for 10 years (or whatever it ends up being) - I'll expect to be treated like a Gold (and as UA decides how they'll treat Golds). It's pretty simple.