The thanks button is about as low as you can get on the "serious situation" hierarchy.
Get a grip John.
Everything is relative.
It is not as important as the things you mentioned, but it is more important than many of the things you don't mention, like getting lumps out of the cream of wheat. It is thus nowhere near as low as you can go.
When you consider relative seriousness, you must consider it in context. When you are running a discussion board, seriousness relates to the continued existence of the board. AIDS in Africa is more serious, but it is out of context. If one were to use your approach, no one would bother with anything they do at work because it is not as serious as mass murder.
Not long ago things were going very badly. You may not have noticed, but a number of once prominent posters, including a couple of moderators, have left SB entirely. It was getting very ugly. A number of solutions were discussed and enacted. Eliminating the thanks total was one small one. It may not have been the right decision, but it was part of a much larger series of decisions trying to deal with a problem that was, in the context of SB's existence, very important.
And by the way, in the spirit of the staff process, I am not going to say what my personal position was. For all you know, and for all I will reveal, I may have been totally opposed to it. I am just saying that a decision was made in the hope it would HELP solve a much larger problem that was tearing us apart. Maybe it was the right decision. Maybe it was the wrong decision. But that is why it was made.