You can always find ways to monetise an operating system. Some possibilities include:
* add products/referral links to os search results (like ubuntu lenses)
* add ads to the base operating system (like kindle's sponsored homescreen)
* sell spots for default search providers (like bing for siri)
And you target old phones by providing them with an upgrade path to cyanogen mod based on a more modern android version. Many devices are technically fully capable of running modern android versions but have been abandoned by their manufacturers.
* add products/referral links to os search results (like ubuntu lenses)
* add ads to the base operating system (like kindle's sponsored homescreen)
* sell spots for default search providers (like bing for siri)
And you target old phones by providing them with an upgrade path to cyanogen mod based on a more modern android version. Many devices are technically fully capable of running modern android versions but have been abandoned by their manufacturers.