A pastebin link (because I'm not a fucking a shitter)

What they're doing to mercury and emerald is what all of peace walker and mgs5 is to like the end of mgs3.
When you see Snake refuse the handshake from the president, everything that you need to know is implied in that simple action. Sure it's neat to how he became to be a giant mercenary warlord, but thats not important in the grand scheme of things. What is important is the moment where he expresses that disdain for his country that is the actual birth of Big Boss. Everything past that is a formality.

In this case, the reason that that sequence works, is in the simplicity of the answers or rather how they're presented. You're not told it, but rather you're given the initial meeting between all of them. With this relatively simple detail you're able to piece together:

A: The reason that mercury is so flippant and seemingly disinterested is because he's not even supposed to be there, and given his fucked up scenario the he was raised by a master assassin who apparently chopped off his legs combined with the fact the only times he's really smiling or enjoying himself in any scene is during fights or inflicting pain, that he's just there because he's a psycho whose been fucked up into thinking that's the only way to get his jollies
B: The reason emerald puts up with all the bullshit that she's complaining about in the all the volumes is because she was a pickpocket on the streets who had nothing until Cinder came into her life.
So when those scenes cut to black before Cinder actually says her offer or what Em and Merc want, it's because the audience should have already come to the answer by themselves and instead of just explaining things you already know, the show respects your intelligence as an audience and doesn't repeat itself. It makes the audience feel like they have a real understanding of what's going on and therefore be more willing to pay attention incase moments like this arise.

Let's say M&K go back to that and further elaborate on that decision. For the sake of argument let's assume that Emerald literally gets to never be hungry again or Merc is never bored again is what Cinder promises them, or that “WITH ALL 4 RELICS YOU CAN WISH FOR WHATEVER YOU WANT”, the reasons that you can rationalize from the original flashback episodes are still valid, it's just that they have a more concrete end goal. Which is nice to know, but isn't essential to know and doesn't really tell us anything new about them.

The only way that we'd learn something new is that what we were supposed to gleam (and could have humanly possibly gleamed) is just completely wrong and is straight up wrong. They're actually after something completely different. Now unless it's done really well, that renders their actions in previous volumes and the original flashback sequence that we’re supposed to get this observation into nonsense barring if said new revelation completely recontextualizes everything they've done. That's something that has to be on par with “Emerald and Mercury were secretly the biggest heroes of the franchise” levels of completely flipping and explaining their actions. So unless that previous flashback is a deliberate red barring, pretty much every possible outcome ruins this nice little scene or the new depth doesn't fucking matter.