A lot of people would buy a xbox one for Halo 5. There is a very good reason for it not coming to 360 and it's called graphical limitations, Halo 4 pushed the boundaries of what the 360 could do, switching the big open enviroments from Halo 3 and Reach to the small detailed enviroments of Halo 4. Examples of why porting it to 360 would not be a good idea are games such as Shadow of mordor and Dragon Age: Inquisition, while both games run on the 360, they are tuned down so much that a lot of the defining features in those games have been cut out or downgraded, the nemesis system in Shadow of Morder is a lot less diverse than on the xbox one and the game runs at around 20 fps with frame drops.
Halo 5 will not be out for at least another 9 months and if so the xbox one would out for approximately 2 years, the 360 while still being supported will be over 8 years old and is just not a console capable of keeping up with the current gen.
Titanfall itself is btw a franchise and is a pc and 360 game as well. Forza is one of the xbox exclusives which is actually a rather big franchise.
The xbox one already had price drops, I bought mine for 300 and this included a copy of titanfall, for 65.
The 360 is on it's way out.