It's so weird seeing how ill-informed so many people on this site are considering the sales/popularity of video games are the main reasons to come to this site over others.
1. Halo is a much bigger franchise than GT even if most of its sales come from the US. It doesn't depend on bundling to sell 10m+, only GT3 would've achieved such sales without bundling.
2. People point to Halo 4 saying it didn't help the series maintain high public interest or whatever, the last time I checked it sold around 9m at retail (with very little bundling) and has probably sold between 10-11m including digital. It's still huge.
3. No, Halo 5 alone won't save the Xbox One. The only franchise that would be big enough to save a console would be an exclusive GTA game, which will never happen.
4. How can we say the Xbox One needs saving already? It'll do fine I think after a price cut and more worthwhile exclusives come along.