I can, unfortunately, provide some insight here...
Some of you may know about this from the parent FB groups, so my apologies for any duplication.
Last Saturday (the day before our first scheduled plebe phone call), my phone lit up w/DD's caller ID. Uh oh, I thought. This can't be good. Well, it wasn't. Turns out that she had a near-fatal cardiac episode in the Hall. I'll skip all of the details here, but she ended up spending a week in the Anne Arundel Medical Center and was released last Thursday and forced to return home.
Thank God that her life was saved, but sadly, she has been deemed her “non-commissionable”. So, after years of hard work (including a USNA Foundation prep year), making the cut and earning her spot in the Brigade, swearing the Oath of Office, and beginning (and crushing, according to the detailers) Plebe Summer, it is now gone - poof! To add insult to injury, we found out yesterday (7/15) that her competitive rowing career is over as well (she'd been recruited by all of the "big" rowing schools and, perhaps naively, thought that may still be an option in her new non-Academy life). She would have gladly dropped rowing in an instant in order to serve, but she didn't expect to have to drop it in addition to having to leave the Academy.
After leaving the hospital, she had to sign her separation papers. Any of us would be blessed to have a fraction of the strength that she had to muster in order to put pen to paper. I can’t imagine how hard that moment was for her. Today, she is having surgery to put in an implantable cardioverter defibrillator (ICD) - at 19. She absolutely did not want to leave. She wanted to go Marine ground and fight the fight. From what was almost her deathbed, she was pleading with the O-6 Brigade Medical Officer for any opportunity stay and fight. We know that in the grand scheme, many are much worse off and that she'll have a relatively normal life. Tough for a 19 year old to see past what has been taken away, but we know she'll get there.
So, to the OP's question - there are worst-case scenarios that would "prevent deployment to austere environments", thus rendering one non-commissionable. If it happens later in the Academy career, presumably after 2-for-7, there are rare opportunities to graduate but not commission. Earlier than that (especially in Plebe summer) it can lead to separation. For less severe scenarios, where one would still be deployable, but just not able to complete Plebe Summer, as others have said, that Mid would simply be rolled to next year's class (assume all else remains good to go - academics, character, etc.)
As a side note, we’ve gained some insight into how hard the detailers work and how much they care for our Plebes. They were rotating shifts all week to sit by DD's bedside 24/7, and told us that they considered it an honor to do so. These kids (yes - detailers are still just kids) are amazing and are by far the hardest working people on the Yard during Plebe Summer. What a fantastic leadership experience for them. I assure you, they want only the best for the Plebes. We are so thankful for the medical team that found the issue and kept her with us, and for the amazing people at the Academy. The respect and honor that they have paid to DD has been truly incredible.
This community has been great as well. If I can every help anyone with perspective on the Foundation program or, unfortunately, the medical separation process, please do not hesitate to reach out.