The App Store on iOS 9.3.5 still provides value for reinstalling legacy apps and basic browsing, but its usefulness is steadily declining. For limited, offline, or legacy-specific use it remains serviceable; for everyday modern app needs, security, and feature parity, replacing the device is the sensible path.
Note: Newer devices, such as the iPhone 5 and iPhone 6, were able to update past 9.3.5 to iOS 10. appstore ios 9.3.5