I add not trusting only the Wires guy but also the Eel: when Simon returns with the info downloaded, and it has Ava's sub in its jaws, the Eel says that Ava was going to download the data like he did and leave him to die, but regarding everything, I also don't believe this. She said that they had success in similar situations with towline broken, and realistically they NEED to have means of pulling SMs up if the towline breaks for whatever reason, if only because each one is a lot of resources. Simon found only one SM, the 8, that wasn't recovered because it was in a cave system, but anything not in a cave system is fair game.
I do think Ava was being more reckless for some time already, slightly rogue on COI council regarding when to send them info and so on, we see, and also considering what David says, but David himself says that Ava takes care of her crew.
*Meta Narratively*, he wasn't supposed to make it back, yes, but that wasn't COI's plans. Ava and David were a bit of dicks to Simon at the start, yes, because of his involvement with Filament Station and later for blasting them with radiation (they're all very human for all these; we all point how human Simon behaves in the SM, but David and Ava's behavior and treatment of Simon, of not wanting him dead but still, well, bullying him because of who he is, is a very human thing), but killing him *LIKE THIS* wouldn't serve them in any way, shape or form.
Also, just after Simon takes the pic of the skeleton and the SM-13 is shaken around? They immediatelly start pulling the SM-13 up and Ava is there in the radio screaming if the SM is damaged because if it is, Simon *can die*. And before sending him back down, David asks Ava about giving Simon more oxigen. Because it was supposed to be short (30 minutes) and expecting to be doing tests on the sample before any other expection, she was like "no", probably because Simon would them be taking out. And per David's words, we know *air* is a worry for the COI, so he being the one to ask about making sure Simon has more oxigen is poignant.
It never was the "plan" for Simon to die. He wouldn't be *missed* if he did, not at the start of the movie, but it wasn't a desired outcome and they were doing what they could to avoid it and the only reason the SM-13 towline was cut was because it was Simon or the whole towship and everyone manning it.