Feasibility of converting long body carriers into stationary field hospitals
This is a very opinion based question that will garner many opinion based answers. Not the type of type of fact based question that is the purpose of this site.

The pros of this concept would be:


Provide revenue for the airline.
Provide space for the ill.


The cons of this concept would be:


Aircraft get awfully hot or cold when not being powered.
There are no adequate facilities for long term waste and hygiene.
The air in an unpowered aircraft would be awfully stagnant. That is not good for the health of the recovering, nor that of the medical personnel.
If worse came to worse, there are a lot of unoccupied buildings that can be rented at the same or less cost than an airplane. They would already have the infrastructure for power and plumbing.
The Army and Army National Guard already have most of the tents and other housing facilities bought, paid for, and in storage for these types of emergencies.
Even for a pilot, timeframe for recovery, stuck in a fuselage, with that many other people would be claustrophobic. Think of how it feels if you are under a stay-at-home sequester order, now. Now, multiply that by ten.
Costly decontamination post-pandemic.
Running an engine or APU 24/7 would be cost prohibitive.
Having lived in a space with fifty other guys for an extended period of time, even with bunk beds, I can tell you that it would require more space than you might think. 1000 square feet (100 m^2) would be my estimate of the bare minimum without dining nor food preparation facilities, and minimal toileting facilities. That would be 20 square feet per person. That is less space than a king sized bed per couple for the entire living space.
Cruise ships and hotels would be a more feasible option.