I had a 2011 MBA and it was a great computer but I upgraded in 2013 and there was a huge difference. The 2011 only has USB 2.0 which really matters if you use external drives. For example, I have a 1TB Samsung USB SSD. Connected to a USB 3.0 port it will transfer data at over 400MB/sec. Connect that same drive to your 2011 MBA and you will only get about 30MB/sec.
Then there is the old wifi on the 2011 MBA. Starting in 2013, the 802.11ac wifi on the MBA is at least 5 times faster. This makes a big difference if you access shared drives on a local network.
Arguably the biggest shortcoming is battery life, starting in 2013 Apple switched to more power-efficient processors that deliver close to twice the battery run time on the MBA.
Now many people might not use their computers in a way that they would notice these things. I definitely did however, and am glad I sold the 2011 machine back in 2013. If, as you say, you're still happy with the 2011 then just keep on going with it. Also there is no law that says you need to install every new version of MacOSX. I am still running 10.8.5 on a couple of my machines.
