This is nothing new.
Microsoft always EOLd support for an older service pack / RTM release a short time after the newer service pack came out. The latest service pack has always received the full 5+5 year (mainstream + extended) support.
The only difference with Win10, is that "Service packs" are now "milestone releases" and Microsoft plans to continue release them continuously (~twice a year). Windows 1607 has been superseded by 1703. This will, in turn, be superseded by the next drop (Redstone 3), etc.
The problem is that some hardware that may be running 1607 suddenly "cannot run" 1703. This needs to be investigated. I am not sure there were cases in the past where fundamental hardware incompatibilities blocked a Windows service pack, when the previous release (of the same OS) ran. Maybe there was something between Win8 and Win8.1?
Thinkpad 25 (20K7), X1 Carbon (20HQ), Yoga 14 (20FY), T430s (IPS FHD + Classic Keyboard), X220 4291-4BG, X61 7673-V2V
T60 2007-QPG, T42 2373-F7G, X32 (IPS Screen), A31p w/ Ultrabay Numpad, A21m 2628-GXU