The question is, once you speak Thai very fluently, will Thais want to speak to you in Thai? It may turn out that however better your Thai is compared to their English, they will refuse to speak Thai with you. Instead of pursuing the conversation in Thai, they will abort the conversation with the usual "How long you been Thailand? You speak Thai good". That's as far as spending years learning Thai may bring you: "How long you been Thailand - You speak Thai good".