How does the Army plan to support mission while getting rid of an MOS?

SPC (Join to see) use your CAC and log onto the branch proponent mil suite and see what doctrine/MOS change is going on. They'll merge MOSs and reconfigure doctrine. Perhaps under this new DHA concept, a joint capability will be leveraged.

There must be a proponent decision to the CSA for major DOTLMPF change like this.

There is nothing I found that 68D is going anywhere. USAREC still has the MOS for recruiting. But I'm an old gray beard and not inside the AKO JKO sphere anymore.

They've been saying that for years, it will he years before it happens as well.
A few years ago, back in 2013 or so, those MOSs were ASIs of 68W, much like F2, flight paramedic, falls under 68W. The people who managed these ASIs came to the conclusion that it would be easier to manage them and keep them in the hospital if they had their own MOSs. After all, why train someone for a year to be an LPN or a Cardio Cath tech for them to go back out on the line to be a 68W later?
What really happened is that the promotion rates for those jobs plummeted because they are ultra low density MOSs. I don't know of that's the particular reason that AMEDD has proposed to merge the MOSs again, but I'd bet it's a substantial factor. There are other factors that probably contribute to it as well. If they become an ASI again the branch manager can control who comes into the ASI because they review and approve all the training. When it's an entry level MOS you have to accept whatever genius or idiot the MEPS puts in that seat. By already be 68W qualified you are ensuring that there will be less failures. By requiring letters of recommendation from the chain of command you can ensure the character of the soldiers you place into these sensitive positions is of a high quality. In the higher ranks you don't need an LPN to be in the hospital. They can perform just find as an NCO on the line. That opens up more positions for promotion by combining the two together.
So, while it may seem like a strange change to you, AMEDD has been splitting and combining hospital and field medics for decades. When I became a medic in 2006, the Army had just combined the field medic and hospital medic (91B & 91C) into combat medic (91W).

With the reverse PROFIS process for TDA assigned 68D's, the changes from CSH to Field Hospitals and the FST's the MOS will not be going away. I have heard about the MOS going away since I re-classed into it and the the only change I see is that there will be more TO&E assigned 68D's compared to current allocations.