Apple - Is it required that I upgrade to Sierra

No, you are not required to upgrade to macOS 10.12, Sierra. While the upgrade is free, it is not compulsory.

Many, including myself, recommend waiting before upgrading. This additional time after a major macOS launch gives time for problems and regressions to be discovered.

You're indeed not required to upgrade to Sierra and you can run El Capitan for years. But the upgrade is preferable because the new version can include new functionality, bug fixes, improved security etc.

I have a bad experience with upgrading from Maverick to Yosemite (I upgraded as soon as possible), when the system was suddenly slower then before. It was better after few weeks when some updates were released. So my current strategy is to wait 2-3 months after the new Mac OS release until major problems are discovered and fixed, then upgrade.

it will eventually be, sooner or later, so i wouldn't delay, myself.

OSX 10.9 is no longer receiving security updates: support.apple.com/en-us/HT201222

http://www.computerworld.com/article/2950580/operating-systems/the-end-is-near-for-os-x-mountain-lion-support.html

by the way, siri on mac is actually pretty good.