This is yet another aspect of A90 where the reality of its BMW roots must be taken into account. With very few exceptions, BMW operates on a seven year model cycle, with model facelift done at the four year mark. Any future Z4 and Supra development will remain joined at the hip, so if BMW runs Z4 through the usual life cycle, that's what you'll also see on the Supra side.Four years?
It should be noted that BMW's schedule of engine and transmission revisions is almost fully disconnected from model lifecycle. While they usually won't put new revisions of their engines into the models that are on their way out, everything else remains possible. Plenty of BMW models had their engine lineups significantly changed after just a year or two of production - whenever new engines or transmissions are ready, BMW puts them into all the cars that can use them. However, the lack of a manual in Z4/Supra is not the question of engineering readiness but rather a marketing decision, and those are only revisited for new models or facelifts.