The difference is that if you still own a Gamecube, Wii, PS2, and PS3, you should be able to play the games you originally bought for those consoles with no issue. I should know, because I do that all the time. Meanwhile, the original Xbox, a console which relied more heavily on online, had retail games like the Halo 2 Map Pack and Counter-Strike that relied on online for their value and are a lot less meaningful now than they were at release. The later you go with generations, the more you'll see this become an issue, though it hasn't come up as much yet because we're not far away for it to really become an issue. This is what is meant by preservation.
Now, it remains to be seen how this will all be a problem in our lifetimes. It's totally possible Microsoft could be so good at supporting legacy online for current games that it actually outlives, like, current Switch hardware being able to turn on and work. But a lot of physical Switch games function just fine without an internet connection, and if stuff like the NES and Atari 2600 are any indication those cartridges could last at least half a lifetime.