|No one can seriously argue that this is a better site format than IV.|
I just realized that you have been a member on both sites for years, so you ARE in a position to judge which format is best. In contrast, I've been staring at the same format for several decades so it obviously suits me well :)
The design here was originally implemented as a "flat" system, in an attempt to keep things simple. You are right that it does tend to assume that people want to read every message. That worked great 15 years ago when people put a lot more time, thought, and detail into each individual post, respecting the fact that it took several seconds just to load each individual post.
Unfortunately, in part because bandwidth has sped up considerably, message boards have devolved over the years into hybrid "chat" lines where each individual post has less content. Some people on SI have posted more than 100,000 times. I understand the need to be able to extract what interests you most.
In summary, I am not opposed to evolving SI's format to fit the times. Could you live without the explicit "subject" line, if there was a way to view an entire "thread" of responses to a message on one page (i.e. not just replies, but replies-to-replies-to-replies-etc...)?