I asked myself and the forum the same thing a few weeks back as I had some projects upcoming where I needed the extra depth and oomph of the larger saw.
An upgrade to this saw would be very welcome - the features that Svar points out really make the REQ a significantly more enjoyable saw to use IMHO than the older TS55. However, even if there was a new TS75 coming, it would first hit Europe, and then get to the US maybe 6-12 months later. So really, you're looking at at least a year before you could have a redesigned 75 in your hands.
I think the 75 users are a small fraction of Festool's tracksaw market, so there's not much incentive there for them to upgrade the saw...they'll either phase it out and decide they don't want to offer a larger tracksaw, or they'll upgrade it to be on par with the TS55R when they run out of other stuff to do.
The decision between the two saws really should come down to what you plan on cutting, and less to the creature comforts lacking on the 75. If you're going to do a lot of hardwood slabs, edge jointing 8/4 stock with your tracksaw, trimming interior/exterior doors, then get the 75 and live with the slight shortcomings. However, if you're going to be dealing with mostly sheet goods with the very rare big cut, the 55 is the better option and you'll figure out workarounds for the rare heavy cuts. The 55 is much lighter, more maneuverable, more portable, etc. The 75 is a big fella, and one must take more care in using it as the kickbacks are more significant and it needs more rail at the beginning and end of cuts (hence why it comes with a longer rail).
Or of course, you can get both and have the best of all worlds. If you do that, get the 55R in cordless form. [tongue]