64 bit has nothing to do with speeding up the game or multithreading.
Perhaps speeding up the game was not the main goal for developing a 64-bit DF. But your statement is misleading. A 64-bit app has
the potential to be significantly faster than a 32-bit counterpart because it allows programming tricks to do things faster. This includes reading and writing 64-bits of data in a single instruction and letting different threads manipulate a shared variable at the same time. And, yes, a 64-bit app has the potential to take better advantage of multithreading and multiple cores than a 32-bit app.
The development of a 64-bit branch of any software project brings
an expectation that it will be superior to the 32-bit version
in some way. It should be more secure, more stable (i.e., less crashes due to running out of memory),
run faster, or perform better in some way. Otherwise, what's the point? The move to 64-bits (not to mention maintaining separate 32-bit and 64-bit versions) is extra work.
In the case of games, especially, a move to 64-bits is almost always done for either better performance or access to more than 4 GB of memory. Usually both.
The fear was that using 64 bit would actually slow down the game. Which thankfully doesn't seem to be the case.
If the 64-bit version was significantly slower than the 32-bit version, I doubt many players would use it. Again, what would be the point?
From 34.11 to 42.xx, there have been
reports of a huge drop in FPS and FPS Death is now much more common. And, as more and more features get added to DF, this situation is only going to get worse.
Just giving pathfinding a separate thread, alone, should see a
huge boost in performance. It's been more-or-less proven that pathfinding in DF is the single biggest performance hit.
...It would require a lot of the features to be rewritten, like pathfinding...
Pathfinding would have be modified, sure. But I seriously doubt it would have to be completely rewritten merely to migrate it to a separate thread.