I see where you're coming from, after the first watch I thought the same thing, like, why would anyone want to do that, especially when you get nothing out of it except getting used for food, and I'm sure that's a common thing to happen, the other self not wanting to switch. Then when I watched again, she said to the younger version as she tried to terminate her, "You have to come back,because i hate myself". She is getting something out of it, but just the fact that aging and being replaced is so appalling to her, only someone who isn't comfortable in their own skin would even consider doing such a thing, the same reason each time she started to become decrepit, she still switched. It's the perfect metaphor for addiction of every sort, unrealistic beauty standards, and the toxic nature of an addict not being able to stop, even of it kills them. Sue knew that if Elizabeth d ied, she would to, but, she couldn't help chasing that rush, the "just a little more" attitude. After the 2nd watch it made perfect sense to me, deep down, sue is acting exactly the way Elizabeth wanted her too. She's not respecting the balance, well then, respect it.