NOTE: Now before we move forward here are a few things we like to share apple macOS upgrades are an essential part of security when you do not upgrade macOS your device is more prone to attack.
Now here are the top 5 reasons we have not updated our macOS since 2015
Compatibility: The macbook air we are using is a 2014 and has an Intel i5 processor, 8GB RAM & 128GB of storage. Now, this hardware is perfectly compatible with our macOS 10.11.6 which is (El Capitan) and I have never had any issues with heating, battery drain, or the fan spinning at full RPM as the macOS is very light for this hardware and the hardware can very easily hold the pressure of this device. We have hardly shut down the device in the past 8-9 months it always goes to sleep and never experienced a crash hence we decide not to upgrade any further. We deal with many apple macbook air for repair from Mumbai users each day and most of them have a complaint about the overheating & freezing post upgrading to the macOS Big Sur also there is a fall in the battery backup of their device
Old Apps: With every new macOS the compatibility of the old apps goes and one had to buy new apps for the same as of now we are using apps like Photoshop CS6 & 2014 Autocad which work smoothly and never give us trouble, Yes there are fewer features in the app when you compare with the latest version but most of our work can easily done on these old apps and hence we do not have to purchase new apps from the app store with each update you may face issues such as not comparable and forced to buy a new version of the apps for the same use we are still using 2016 Microsoft office all these apps were purchased long back and we are able to enjoy it without paying anything extra. Yes, we do not have many features which the latest macOS has such as dark mode, apple music, etc but as my work is more related to emails, word pad & google chrome nothing to worry about.
Storage: As I said this macbook air has 128GB of storage which is still 30% empty it can easily allow me to save lots of files for the next 1 year as you must know then any updates apple releases easily consume space from 4GB to 12GB which can make your storage full by the just release of 2-3 updates. Yes, there is always an option open to upgrading your SSD on the Apple Macbook Air 2014-2017 year which can be done by any macbook repair center in a few mins.
Heating: The new macOS comes with lots of features and these features may not be useful but consume lots of hardware resources and which put a heavy load on the CPU which many it hot and the fan has to do all the work of cooling down the device.
Battery Backup: We have also noticed the battery backup drops when you upgrade old MacBooks to a new version of macOS.