To do that within the scene wouldn't be appropriate with Roswell grieving over losing Thanatos though. While in supplementary material Oswin is lenient and lets it be Roswell's choice, there isn't room for it in that scene and by the time we've moved on, Roswell isn't present to deliver in dialogue what he wants.
Yeah, unless Thanatos deactivation could be done at a slightly later point. The vault might also not be so deep Oswin revealing the medicine would be out of earshot. Either way, i can presume the discussion with Roswell to happen off screen. But, i still feel like it would be more organic for Roswell to bring up the option rather than Oswin. I guess the only way you could show that is if later at the hospital or the vacation, for someone to comment that this conversation did in fact happen. But that's about it, i don't see many comments concerned with this specific point, the ending might be just fine as is.
It has its problems, but just a lot happens in a short span of time and a lot of things depend on other things. The vault opening kills Thanatos. So Thanatos cannot just go offline later if we want any sort of view inside of the vault; which if we don't do that then we'll have people always wanting to have seen what's inside in the events of the game properly. At the hospital, Roswell still hasn't decided if he's taking the medicine because he hasn't spoken with Oswin properly as far as the risks. If we move that conversation forward then him grieving Thanatos and the loss of his friend and confusion about what's happening alongside Dave telling him he needs to seek help is all lessened to have a monologue or side comment about him making another choice. The point was that Roswell was not in control anymore, the choices aren't his to make in the game anymore, he needs to let go.