If Halo Infinite is having problematic development, I don't think they should rush it out to hit the Fall 2020 XSX launch, even if it has been in development for ages already.
Halo is Microsoft's biggest exclusive IP, and I think a new Halo that misses the mark or is critically lacklustre, could have really negative mind share, branding and marketing ramifications for not just Halo, but Microsoft's move to the next gen and the perception of their potential output in general, especially if the competition is releasing huge critically acclaimed exclusives in the same year.
That said, I'm still fairly confident Halo Infinite could be something quite special. It's been in the cooker for an uncharacteristically long time, had an entirely new engine developed for it, and we've heard it's a re-boot of sorts, hence could be really ambitious. The only negative for me is that it's cross-gen, but that doesn't mean it can't still be really impressive on the XSX, especially if they don't prioritise the Xbox One version much.