The OT has something of an apocryphal history and status. Someone can likely correct me, but from what I can piece together: the main mastermind behind its programming passed away and another one of the main contributors now works at a different company. (ASM?) This has left them with a deeply personal codebase that they can tinker with but not overhaul in any significant way. *
Elektron has proven that they have some great skill with the current Digitone / Takt lineup, and I have to imagine that if there is a succesor it will be some kind of high-end box that is done in the image and codebase of those two rather than an Octatrack upgrade.
People on the boards are speculating everything: itās a 10-year anniversary model, itās being retired, itās being upgraded to a MKIII, itās being upgraded but only with a black faceplate to make the DT/DN, etc.
I would definitely wait, but I was seeing all the same questions and speculations before I bought mine a year ago. Iāve had it a year and still like it. 
*Since moving into management roles, this is why I always stress how some individuals have a high ābus factorā. That is, how screwed would we be if this individual was hit by a bus tomorrow?