If you can sell or give the old phone to someone else, they are also displacing a non-fair phone. So filling the market with fairphones (That are still supported) is a GOOD thing. And you are still supporting the company, and all the people they pay as well.
In general though the best thing is to just stick with your current phone if you can. Every new phone still consumes resources to produce and probably replaces some other phone which then has to be recycled or might even end up in a landfill.
If you can sell or give the old phone to someone else, they are also displacing a non-fair phone. So filling the market with fairphones (That are still supported) is a GOOD thing. And you are still supporting the company, and all the people they pay as well.
In general though the best thing is to just stick with your current phone if you can. Every new phone still consumes resources to produce and probably replaces some other phone which then has to be recycled or might even end up in a landfill.