She had health issues that made it impossible for her to continue. Hopefully her health will improve, but if it doesn't, she may not be capable of doing a job that involves using her voice in the way that a vtuber does.
She went to school and spent the time and effort required to earn a degree. If she wasn't planning to pursue a career in the field in which she earned that degree, then it was a waste of her time and money. I don't think that Mumei was planning to make vtubing her career. I think that she was always planning to finish school and move on to something else. I think that she just saw vtubing as a fun thing to do to pay the bills until she could finish school and find a job in her chosen field. Not everyone is planning to be a granny streaming from the retirement home someday, and that's okay. I'm sure that her time in Hololive will always be a cherished memory for her to look back on, but maybe being a vtuber isn't what she wants to do with the rest of her life, and that's okay too.
If she was earning as much as you seem to think she was, and she invested wisely, she probably doesn't have to worry too much about money for a while. If nothing else, she probably was able to repay all of her student loans. She may genuinely be happier doing some other sort of work, even if it doesn't pay as well.