Technology

I absolutely agree they can maintain an AOSP fork going forward, and I think that's completely realistic and I would be surprised if that is not the case.

But I was answering OP from a strictly technical perspective about the potential difficulties they could, theoretically face while doing that. Since you asked what is the hard part, I'll answer along those lines (again, with the caveat that I don't think these are going to pose realistic obstacles for the GrapheneOS team in the near term) My point is not to say it's impossible but I think it's important for people to be aware that this approach comes with risks, and those risks will grow over time especially when you're up against a non-cooperative upstream that is one of the largest and richest tech companies in the world.

For one thing you're never going to support any new phones without either pulling driver support from AOSP or reverse-engineering the hardware and drivers yourselves, or accepting that some parts will just... not work. So you get stuck on older and less capable hardware. Maybe you don't care about that too much, and that works fine for awhile, but eventually the cracks start to show. Now you have to either start figuring out how to get into the newer hardware, or you have to start getting custom newer hardware of your own, which is $$$.

Using closed hardware this way as leverage is a pretty common way of getting in the way of open source development, and Android hardware is very closed. Similar tactics are already even being used against x86 PCs now with things like TPM and Secure Boot. It doesn't completely brick your system on day one of course, but the erosion of support begins when they start writing software that intentionally relies on these features to say "oh, sorry, this software you want to use? it won't actually work on the open source OS/open source client because they don't have access to this hardware... what a shame." One or two pieces of software, no big deal. But they won't stop there, eventually it'll be like half the software, then over time it'll become 90% of the software, you won't be able to find alternatives. They can often afford to be more patient and relentless about this shit than we are. The battle will continue, and there's no sure path to victory. Forking is one tool we have, and that's great, but we also have to remember that it's not a flawless, unstoppable long-term solution that we can play as a trump card whenever corporate interests do something bad. They don't just give up. They have other means of getting their way.