if the BA button was replaced by a " THANK YOU " button, it might register better for the op, particularly newbies.
I almost agreed with you there ... until I thought about it some more.
Best answer is quite specific -- there can be only one best answer in each thread. That is part of the problem, because in many cases, especially where the original poster did not express the problem clearly and concisely, there could be several replies that all contributed to the eventual solution. Yet only one of them can be given best answer, even if two or more posters separately answered different parts of a problem.
Thank you sounds like it ought to be a solution ... but it also sort-of implies you should be able to set it to more than one reply. If the original poster asks an unclear question, then a respondent can be thanked for asking the right questions, probably leading to somebody else being able to answer the question more clearly and concisely than if they weren't sure what the question meant. In that case, both of them need thanking.
But in real life, a thank you can apply to a very wide range of circumstances, to the extent that if the button could be used multiple times, then it would probably become customary for the OP to apply it to almost every reply in the thread. That in turn would progressively make it meaningless, because there would be no point in duplicating all the replies at the top of the thread -- it would not help other people with the same problem to get an answer without reading the whole thread twice.
So, though I think there's a lot of merit in the idea, in practice I don't see it achieving any more than the present method.