Mark my words: You won't even have the option to play a new (enough) game at low quality on an old Xbox. It will be a seemingly arbitrary cutoff (2018 models can't run it, 2019 models can) put in effect to stimulate upgrades.
So say I buy a launch Xbox and keep it without upgrading for 2-3 years. I buy a online multiplayer game in the 3rd year, would I be screwed against people with the upgrade Xbox?
Listen to what Phil said directly. They are talking about an updated xbox one. Which means what? It will play games that are XBOX ONE games. The comparison to the pc is only partially apt. PC gaming has gotten along with targeting huge amounts of graphics cards. This will be 2 or 3. And even then probably only one or two components will change...gpu and/or cpu. So probably the next will support a slightly higher texture res...like difference between medium and high and perhaps a different frame rate or res