Writing as a third-party developer, supporting older platform versions adds cost, adds testing, and reduces what new features can be adopted, opens up the app to security or stability problems since fixed, and that investment and that risk for people that are either not maintaining their devices to current, or for people that are using older and now unsupported devices. This is not a free trade-off.
As part of this, some of the major third-party vendors have a policy of supporting the three most recent iOS or iPadOS versions.
Apple customer usage of older versions drops off pretty quickly, too; customers do tend to upgrade fairly quickly.
If you do decide to replace with a newer or new iPhone, the minimum hardware for iOS arriving later this year 16 is expected to be an iPhone 8.
This older iPhone can be used as an iPod touch, if you are not ready to retire it.