The steam platform doesn't change like Xbox. Steam is steam, XBox 360 and Xbox one are not the same.
You are absurd to think they won't shut down their servers. Why wouldn't they? You think microsoft is going to keep their servers up in 15 years?
Well damn, I need some help with my Windows 98 system, but microsoft removed almost all their knowledge base stuff for Windows 98 on their website. Why? I mean, was there a need? They just want you to buy their new stuff. So they say "Your system is too old so fuck yourself, we won't help. Want help, get a newer version of our product."
Same for Xbox, they aren't gonna give a shit about Xbox one in 15 years. Hell, in 10 years even. If they can't keep some extra html documents on their servers to keep the help for Windows 98, what makes you think they are going to keep dedicated servers themselves for Xbox?