RK3399 is completely FOSS, from bootloader, to firmware, to Linux drivers and mainlined. It will be supported as long as someone wants to run software on it.
Raspberry Pi, despite not being fully FOSS, has a much better community supporting it. I’d bet it will last longer than the pine boards. Watched Jeff Geerling’s videos on it and he had trouble getting the pine board to work, whereas the Raspberry Pi worked first try. The only images for it were some images a guy made and put on the pine wiki.