It isn't even necessarily that they burn out. Things can just happen that weren't in their life plan. For instance, my 2nd therapist's son was diagnosed with cancer at one point and he took a lengthy absence (about a year) from work in order to care for his son, take him to another city for treatments, etc. His clients had to find another therapist for a fairly lengthy amount of time. He wasn't incompetent or unprofessional; he was simply prioritizing family above work (which is perfectly natural and understandable). Another therapist took a job in another state. I didn't have much warning, but I always knew that was a possibility and it was change that I worked with. He wasn't incompetent or unprofessional; he didn't "abandon" me. He simply made a decision about his life that was best for him and his family.

Change happens. Any therapist who promises that nothing could ever happen that might cause them to have to stop working is being unrealistic and promising something they simply cannot promise. Clients need to keep themselves in touch with that reality and just be aware that, as hard as change is, it can happen to anyone, even the most caring, supportive, professional therapists.

ETA: That doesn't mean you need to spend a lot of energy worrying about it. Most likely, your T will be with you until she retires or you are finished with therapy. Just also don't live in complete denial that something could possibly happen at some point that you might have to adjust to. Don't obsess about it though. One thing I've learned in life is that life has its own plan that often doesn't jive with my own. It ticks me off when that happens; it upsets my world, but I have to deal with what life throws at me. Being able to adapt to unexpected change is a survival skill. You may not need that survival skill, but it's always smart to be prepared to have to put that survival skill into action.