Hey guys, I've done a quick bit of research here and it looks like this is expected behaviour:
Sierra will not show up under Purchased.
Also:
El Capitan is staying in the App Store for anyone who cannot install Sierra.
This all jives with what people have been saying about Sierra not being linked to the Apple ID anymore.
It's an all too common situation where someone has upgraded OS X and then sells/gives away the Mac, then, when the OS needs to be reinstalled, it can't verify because their Apple ID doesn't have the OS assigned.
Apple even
recommends doing an Internet Recovery to go back to the original OS and letting the new owner/user upgrade themselves.
Going forward that won't be an issue as there'll be no check against the Apple ID during reinstall.
I don't have an article to back this up, but I suspect that El Capitan will be sticking around for a while longer as a jumping off point for the next release or two. Given that the
system requirements hadn't changed since
Mavericks, anything that can't run
Sierra will need to be as up-to-date as possible.