They're not shutting down their coal plants, they're reducing the duty cycle of their coal plants. They have more coal capacity in 2025 than they did in 2024; they're just using them less. So they're not going to have base load and reliability problems.
They can continue on this path forever. One can certainly imagine a scenario where they maintain a coal plant for emergency purposes but never hit the emergency situation that needs the plant. They can have 0% coal usage yet still have significant coal capacity and no base load / reliability problems.
Right now the coal plants run at night. They're adding batteries that will take over those duties, so the coal plants will eventually just run when they have N days without sun & wind, with that N increasing over time until N hits a number that is highly statistically unlikely.