for how long will xbox s be “next gen”

does it make more sense to buy 300 dollars for xbox s instead of 500 dollars xbox x. i am wondering how long it will be until xbox s is “old gen”. if that makes sense. basically, will xbox s always play on the same game mode (next gen vs old gen) as xbox x?

https://www.xbox.com/en-US/consoles/compare

They will both be "next-gen" for the entirety of their careers. One is just beefier.

Theyre both next gen and will keep you good for 10 years. 

The difference is the $500 has better hardware on the inside and you get disks if you have movies or games you want to play on disks(which is why I bought the series X)

I asked several folks about the difference between the S and the X (switching from PS4 to XBox with 23).  As noted above the X offers disk use, a hard drive and better (more powerful) components (been told the X loads up much faster then the S).  The biggest difference was the ability was the X is able to take full advantage of a "4K TV" and the S is not (so I have been told).  Now...I don't have a 4K TV, but I might someday.  So for my money...I ordered the X (should arrive later this week).  I would rather have a capability and not need it, than want it later and not have it (kinda like why I bought a 4x4 vehicle).