I recall seeing some threads in the past about Disney working on a plan to purchase new monorails, but haven't seen anything recently. When we were last there in 2019 some of the existing monorails were being 'refurbished' and updated with new interiors or the exteriors were being repainted. Assume some sort mechanical/electrical work was also being done. Obviously, they can extend the life by replacing things that wear out over time. Eventually the mechanical/electrical parts also wear out and part of the decision to buy newer cars would probably be based on the economics and availability of repair parts/feasibility of servicing them compared to totally new cars.