I'd love to hear feedback on whether people who actually have received thank you letters here think they are a good idea. And I know it's been discussed from an outside perspective, but this thread has a few people who have stated they have acted in some official capacity in the process and their overall thoughts seem to vary not only among themselves but also in comparison to what those outside perspective threads have focused on.
My research advisor and I talked at length about this, as both of us were raised in a very strict military/southern way so ma'am, sir, and thank you come out of our mouths orders of magnitude more than the average person around us. Despite this, he despises thank you letters in all their forms. Courtesies abound, but they are supposed to be genuine or else they have the opposite effect. And what is genuine about 1,000 thank you emails from virtual applicants no one in the committee has ever met and shook hands with?
And even beyond this, to your point, by the time someone might read any of these things their decision has already been made. Now, I'm basing that on the statement you and others have made here, as I certainly have no first-hand knowledge, but it all seems quite cut and dry. And yet, in those out-side perspective threads the prevailing opinion is to always send thank you letters and to send letters of intent every month after your interview. I see it preached here, reddit, twitch, and on pretty much every pre-med guide website and youtube channel.