What if for every prisoner who chooses this path, the equivalent amount that would have been spent on them for the rest of their life, will be invested into a community project for the next 20 years instead, like building and maintaining urban parks and greenspaces in local communities that they affected.

That's a neat idea in a society and in a species that is incorruptible. However, once an incentive is given to end a life, there exists the strong possibility of corruption. One can imagine pressure being exerted on an inmate to choose the "charitable" option. One can imagine incentives offered, such as kickbacks to family members. One cannot imagine all the possible ways the system could be corrupted, but if there is money to be made, it is certain someone will find a way :))