Yes I know it's been custom for 100s (1000s?) of years for the author of a letter to sign it at the bottom. I guess this came about due to security and eventually legal reasons. What the author wrote ends at the signature, so any blank space on the page can't be fraudulently filled in later.
That was a long time ago, and here we are in the internet age with rather different reasons for "signing" a post. This is now simply to let others know who wrote it. This is information you want before you read the post, not after. In today's world, especially here on the internet, the signature should therefore be at the top of each post, not the bottom.
I can't fault SE for following the convention and doing like it's been done for 1000 years, but let's step back and think about this anew, folks. The identify of the author is important context to post. This context is important to know while reading a post, not after. For short posts this doesn't matter as much because the whole post including the signature is on the screen at the same time. Think about it, for short posts do you automatically look at the bottom first to see who wrote it before reading? Do you maybe do this without being aware of it? I know I've scrolled to the bottom of long posts to identify the author, then scrolled back to the top to read.
Is there any advantage to having the signature at the bottom as apposed to the top? If we can't come up with one, then it should be moved to the top.