Well, I do not upgrade immediately either; however, what I have learned is that Apple withdraws the old OS version as soon as the new OS is publicly available. So, even if you do not wish to install it immediately, you can easily download it and tuck the installer away in a safe place - you always need a full installer which you can then easily update. I am still running El Capitan on my production iMac, but I have the full installer of MacOS Sierra downloaded and parked on an external drive until I am ready.
As for El Capitan - it is only available if you've downloaded it before, are running 10.6.8, or your Mac does not meet Sierra's system requirements. It was available to everyone before Sierra was introduced.
So, my suggestion to everyone is: download Sierra and put the installer somewhere safe until you're ready for it.
How to get macOS Sierra - Apple Support