You said Xbox 360 was launched in middle generation, but fact is that isn't true. 360 arrived just one year before all others console of that generation, that definitely isn't middle of generation. NX coming 3 years after PS4/X1, I dont see PS5/X2 before 2019, so it basically will be in middle of PS4/X1 life cycle.
Mario Kart 8 was finished at beginning of 2014, I don't see why isn't logical to expect new Mario Kart game at end of 2017, also Retro finished DK at end of 2013, they are also on track to release their game in 1st year, there are definitely some 1st year projects we don't know anything.
It was the middle of the gen though only difference was the reaction that it forced, both Sony and Nintendo to drop their consoles, 4 years into the life of both the GC and Xbox is the middle of the gen considering average console life is 6 years and again you ignore the Megadrive which highlights this is nothing new.
It's not logical because the's no indicator saying as such, you've just thrown that assumption in to try and fill a hole in your stance, before E3 2015 people swore the were many more new title for Wii U as well. On a shared library like you've said you're going by MK would take a bit longer to develop in order to work for both platforms being multiplayer focused and Retro have yet to do a game that is a big system mover as Metroid doesn't shift consoles so would be pointless as a holiday title and DK only really sells to the userbase already there so again can't fill that slot as a holiday mover.