^ Generally takes about 5 years, yes. But just because the A90 is set to end production then doesn't mean that whatever replaces it will instantly be available. It could also not happen at all given the current automotive climate, but hopefully that's not the case.
What if it was from another manufacturer? Is the most important thing that the engine is from Toyota versus design? What about the platform itself? Say the drivetrain was Toyota and platform was from someone else?I6 of course, but if that means BMW involvement then I'll gladly take the V6.