They already do have AI therapy assistants. CBT type therapy is particularly easy to turn into an app. There are half a dozen in the Google Play store now. They're a nice reminder at times, but no substitute for human conversation.
Once we do have AIs capable of conversation indistinguishable from real human, then therapy is not the only job that will be disrupted. Therapy will be no more or less safe a career path than so many other things.
Second, humans will still need to program, train, and monitor the therapy AIs. The obvious candidates to fill the role at first are experienced therapists with a bit of tech savvy. Until they optimize to the point where the job can be done by warm bodies paid minimum wage, probably "contractors" so liability can be compartmentalized. Then we're back to the point above where everyone in any career is fucked anyway, might as well do what you're good at and what you enjoy for a decade or two.