Xbox isn't even needed to keep Sony in check. PC, soon Nintendo, and later hybrids, and even later mobile phones and TV's will be enough. GamePass is a potential challenger without a console needed.
Home consoles will inevitably die or massively-shrink as a market. PC, mobile phones, and smart TV's will dominate the future of "console gaming", and we will be reminiscing the good ol' days of true dedicated gaming systems.