Think carefully before upgrade. You can't downgrade after you upgraded and new upgrades are known to significantly slow down old devices. My iPhone 4S became unusable after iOS 7 and it's almost bricked with iOS 9. This is Apple way to force users to buy new phones.
That said, they force developers to discontinue support for old iOS versions, so if you don't upgrade, you'll stuck with old apps and some might stop work if they rely on remote servers with changed protocols. So if you want to use iPhones, be prepared to buy new phone every 3-4 years.
Apple doesn't "force" developers to discontinue support for older OS's. Apple does require that you at least have a build that supports both 32 and 64 bit versions.
Apple also will allow you to download an older version of an app if the newest version doesn't support your hardware. I can confirm that this works back to devices that only support iOS 5 - like my first generation iPad from 2010.
Apple occasionally release upgrades which the hardware really isn't capable of supporting. I think the worst example was the iPhone 4, which was given the iOS 7 upgrade. iOS 7 was basically only suitable for multi-core capable devices, and the iPhone 4 was the only device it was released on (the same generation iPod and iPad didn't get the update).
Apple tries really damn hard to make their stuff run well on all devices and it's very impressive how well they support them.
But if your device is a few years old… You're right that you may want to wait and see what the reviews say. Sometimes the updates actually make the phone feel faster, but sometimes (like iOS 7 with all the new transparency effects) it definitely can be a worse experience.
That said, they force developers to discontinue support for old iOS versions, so if you don't upgrade, you'll stuck with old apps and some might stop work if they rely on remote servers with changed protocols. So if you want to use iPhones, be prepared to buy new phone every 3-4 years.