> It must be very deflating to the programmers who came up with this scheme to learn that so many people want to turn it off.

I can see that the scheme is quite clever, but it just doesn't quite fit the bill.

Here's an idea: maybe there could be what we call in my company a "volume control" e.g. a slide bar.  In one extreme position every new message always goes into any existing conversations with the same subject.  In the other extreme position, every new message always starts a new thread.  The default could be just like it is now.

In other words nothing needs to change except for some parameters in this heuristic algorithm.

[Hmm, those parameters would need to be stored and applied for every individual.  But maybe it would be the least disruptive option.]