Earlier quoted context omitted.
The iPhone 5s from 2013 can run the latest OS. The latest version of MacOS supports computers back to 2012. But while the Mac is not a huge percentage of Apple’s revenue, by itself, it makes enough money to be a Fortune 100 company.
Yes, you can run latest iOS. But it is dog slow. Try to use it with at least 3 apps. For a typical user with > 10 apps it is useless.
The number of applications installed doesn't matter. iOS will purge apps if they are running in the background and take up too much CPU or memory. You're not going to have > 10 apps running in the background. More than likely, you won't have any third party apps using CPU time in the background besides playing music or VOIP.
EDIT:
Just realized that you may be referring to macOS. In that case, that would be RAM and/or disk dependent (SSD vs spinning hard drives).