ELI5. Why does the thymus gland atrophy in adults?

Thymus gland is the "school" that makes naive T cells, but the thymus gland is atrophied and effectively not functioning in adults. Especially in cases where it's removed as in myasthenia gravis. So how are new T cells created if they are even created? and does that mean that repeated infections especially those new strains of Coronavirus can exhaust the existing pool of naive T cells in vain? because the naive cells will be primed against every new strains of the virus rendering the previous version of memory cells useless, and the pool of naive T cells is finite and will be exhausted some day. How does it work? Why is the thymus even atrophied when it plays such a critical role. And if the pool of naive T cells is so exhausted what will happen? like would the person get infected over and over with the same new pathogen because the body can't make memory cells of it? I know it's complicated but I really can't understand this.