My wife has and older MacPro that cannot update to the new macOS. What will happen when iTunes is no more? Will she be able to get the 3 replacement apps on high Sierra?
For now, word is that everything will stay bundled together in iTunes on older OSes. I have every expectation they'll be updating "iTunes" for years to come, on both Mac and Windows.
Apple wants us to keep spending money, whether the app where we spend it is called iTunes, Music, TV, Books, etc. So, there's going to be a working store for older versions of macOS and Windows, no matter what name/shape it takes. Further, since iTunes is also the repair tool for iOS devices... they want as many computers as practical to be capable of getting an iPhone up and running again.
For example, my 11-year-old iMac runs El Capitan (two versions prior to High Sierra), and they released updates that support iOS 12.x devices on El Capitan. Windows 7 machines can also run a current version of iTunes. So, figure you're good with up-to-date iTunes for
at least 10 years from the manufacture date of your Mac.
I think there are good, technical reasons why they won't split on older OSes, but the explanations get pretty muddy, pretty fast. Suffice it to say, splitting into three (or more) pieces on the surface argues in favor of shared, OS-based support for certain commonly-needed features under the hood. Again, it's not the kind of change they make to old OSes.