OK, some points have already been mentioned, but now that I've typed this monster, I will publish it
IMO just because a program can be operated fast and conveniently with merely the keyboard should definitely _not_ be a reason to neglect a possible feature completely, just because mainly mouse users could make use of it.
Both, keyboard and mouse are standard input devices nowadays and more or less used by practically every PC user. The goal for a premium-class file manager like TC should be to acheive an optimized interface for both, keyboard and mouse users.
If I intend so do some file managment, I want to do it fast, fluid and convenient and I will use the appropriate input device depending on the current context and personal preference. E.g. if I'm currently scratching my head with my left hand, I will click on the Toolbar button for quickview to invoke it, otherwise I always would press CTRL+Q. It may be fast to operate an application with keyboard, but in many cases it can be even faster using mouse AND keyboard.
So I would say if you see a slight chance, Christian, that you can implement some of the strongly requested improvements into the tree (Quickfilter, DnD and rename Improvements, one tree at the right side...) in TC 7.5 or later, definitely keep the current trees for now as a start. Even if the current implementation has much room for improvements, the sepTrees are already useful for at least some people among the limited amount of beta users. Projected to the vastly greater amount of people that will use the final, there might be hundreds of people that really can and will make use of the trees.
In addition the sepTrees are optional, so if anyone doesn't use them, he will soon forget that they even exist.
_If_ you really completely have abandoned the sepTrees (I hope you did not), I agree with the others that you should remove them now. Introducing the Trees in v7 when you already know that you will definitely remove them in the next release wouldn't be a wise move IMO. It would certainly annoy many users that like to use the separate trees once they are available, and that would for sure result in numerous complaints when v7.5 would be released without the trees. In addition, it will appear as (and actually be) quite a gap in the otherwise extremely consistent development of TC. IIRC this would be the very first major feature that would be completely removed after its introduction.
So my vote would have been: I quite rarely use the current sepTrees (because I got used to the file-list-way over all those years since ol' NC), but would certainly use an improved tree implementation (with e.g. a quick filter) way more often.
And not least, my 2 cents about breadcrumbs:
I strongly agree with Lefteous that it is possible for sure to make a breadcrumb navigation usable with keyboard too: Imagine you could press some shortcut and the breadcrumb-menu for the current directory would pop up at the path edit field. You then could fly through the directory levels by just using cursor L/R, and with UP/DWN you could navigate through the directories inside a level. This definitely would be convenient, unobtrousive and fast!
I think a well concepted breadcrumb navigation could be a real killer feature and would be a noticeable time-saver for mouse _and_ keybord users. There already exist many different breadcrumb-implementaions that could serve as a source of inspiration to implement the best and leanest breadcrumb bar in the known universe. I really think it is possible to implement a powerful breadcrumb bar that also doesn't occupy too much room. And after all, it could be kept optional.