Is it theirs to open up though? When I last looked at it, it seemed as if most of their tech was Ambarella SoCs. Doing a quick search now, it seems they went with Socionext for Hero 6.
Even if they could open it up, it would be a non-trivial cost to support. Multiple products, probably multiple breaking API changes between firmware releases, ... It would probably have to be on a partnering program basis, and for that you need partners willing to share the costs.
As for lowering the price... The market is becoming saturated, competition increased and cheaper alternatives have been popping up. Lowering COGS is hard. GoPro grew up and cornered the market for a while. In doing so, they grew in the environment they were in, but the environment changed and now they have to adopt to the new conditions. That's easier said than done.
I'd say they'd have to broaden their product portfolio in order to avoid/get out of stagnation and decline. I think that was their thought when going into the drone market, but it didn't go well from what I understand. There's other markets for products built around optical sensors (surveillance/security, "smart" sensors w/ computer vision, ...).
Supporting an open developer program is probably not a viable thing for them to do right now.