I don't think you (and many others) understand about hearing loss. There are no grounds for your assumption at all.
Making sense of what someone is saying is a two stage job: Tiny hairs in the ear are dedicated to particular frequencies, and when these are detected messages are sent to the brain, which then makes meaning out of them. As these hairs degrade then the frequencies they pick up on become harder to hear, but they don't all die off at the same time or the same rate.
If specific frequencies are missing from your hearing, your brain is already having to work with limited information. If a particular accent then removes some of that already limited information (for example by not articulating consonants or by speaking so quickly they don't form words clearly) then that accent is going to be very difficult to understand whatever volume it comes at. Getting a better hearing aid might help if you haven't completely lost the ability to hear a particular frequency, it can boost whatever you are still able to pick up. But if the hair cells in your ear that are responsible for a particular frequency are dead, they can't be brought back to life.
If the sound input is less than ideal, the brain will look for help where it can, using contextual cues, lipreading etc. Accents vary in the additional help they give. Some don't move the mouth much and these are harder to understand than those that do.
The situation matters as well, You will have plenty of contextual clues if you're making small talk about the weather, but far fewer if you're trying to take in an unfamiliar topic.
So I can completely understand why someone with a hearing issue would struggle to make sense of some accents in a hospital setting.