I think your take on this is very wrong.
An open source driver written once will work for all future versions of Fuschia. Right? That reduces the effort for open source developers enormously.
Most security flaws come in above the driver level. Right? Being able to update the OS, despite the hardware manufacturer's laziness, is a huge boon for everyone.
Fuschia is open source. Drivers written for one Fuschia (say, the one Google controls) should work for another Fuschia (say, the one you and your friend forked.) Right? That's a huge win for you.
Whatever incentives you have imagined that hardware manufacturers have, time has proven those incentives don't work. It's far better to take as much control away from them as possible. They will do the minimum amount of work necessary (write new drivers). Then we can all exploit the hardware + driver + Fuschia system.
It has always, and will always, be possible to build propriety drivers. This is minimizing the blast radius of them never being updated, as much as possible.
For these reasons and more, we should all hope Fuschia is successful.
The Android developers who have tried to live on top of Linux are sick of the problems that Linux brings them. Fuschia is an open source direct response to those limitations. It's stunning to me that you would wish them to fail.
"then any improvement that could be added to Fuschia could be added to Linux". Explain to me why we still have BSD? Not all problems are solved by Linux. The sooner you accept that, the better off we will all be.
It's like you're wishing Clang would die, because we can always just improve GCC. That's demonstrably false. Just like saying everything good in Fuschia can be added to Linux. No, it can't.
"We don't need a new operating system whose only advantage is that it's easier to write proprietary drivers."
Wait, if it's easier to write propriety drivers, doesn't that lower the barrier to making low-cost phones? Well, Christ, sign me up. Do you have any idea how many billion people have a low-cost smart phone as their first computing device? If we lower the cost even more, that will broaden the reach even more.
Do you seriously not get that the other major phone OS is completely closed source? Maybe put a little faith in the people who have developed the most successful competing operating system that's open source?