I have not and am not proposing for full driver rewrites and customization.
So with that out of the way...

Disagree.
I think you underestimate what the community can do if they were provided with the correct platform, tools and support.
I mean, people customize GPU BIOS's to unlock functional blocks on chips for more performance.

We are still living in a mostly Direct X 11 and OpenGL world right now anyway.

Right-o. Hopefully this incentivises more powerful single cards, despite just pushing the same task onto someone else.

Well Nvidia does not really do much to help the FOSS comunity.

AMD drivers on Linux are pretty good.

Yeah, not only are they inferior to Nvidia (from experience...), but they frequently don't even bother to make any - see the recent AMD on Ubuntu 16.04 and later situation (no idea as to other distros, I assume it's a kernel thing though). Meanwhile, You can take an old Fermi video card and it's gonna have either the most recent or a close to driver on Linux.

Going Linux, go Nvidia for now.

https://www.youtube.com/watch?v=IVpOyKCNZYw

It is generally accepted that Nvidia are not the people to go to for your Linux driver needs.
AMD actively helps the open source community.
I use a RX 460 on my main Linux box and it works perfectly (using Debain 10).

https://www.youtube.com/watch?v=IVpOyKCNZYw
https://www.phoronix.com/scan.php?page=news_item&px=Nouveau-XDC2017

I don't undestimate what the community can do, in fact they can't do anything for the most part since the driver and game code is not available for moddification ... 

AMD are too lazy and cheap to support anything older than your 460 (or most architectures that are not the latest, or near so) on a recent distro/kernel, meanwhile Nvidia has a great prorietary driver, available for cards as old as the GT480 (and probably older..) from ~2010 IIRC. Nouveau is crap, and perhaps it is so because of Nvidia, but there is no reason at all to go Nouveau when you can go proprietary.

Helps the open source or not, it does not really matter to the average user: Nvidia supports more cards, supports them longer, and does not rely on enthusiasts to do their job. Best case scenario these days - https://support.amd.com/en-us/kb-articles/Pages/AMDGPU-PRO-Driver-for-Linux-Release-Notes.aspx , which misses a huge lot of older cards.

I realise this. Which I WHY I proposed that tools and a platform be made available to get around that.

Citation needed.

Yeah ... 

Tools aren't going to help you with that since you need permission from game developers to make their source code accessible to the public ... (doubt they'd do that since it would give absolutely no incentive to purchase freeware and the amount of users that can benefit by contributing are minimal plus there's not much motivation in contributors in improving the source without financial incentives either) 

We're just going to have to trust that AAA developers will implement mGPU on their end ... (it's understandable why faith in this would be low but I don't think 'crowd sourcing' is the way to go about it but if the worst comes to worst then I guess we should 'crowd fund' the feature for AAA game developers to see how truly valuable it is) 

*Face Palm* I am not asking for game developers to make their source code accesible to the public.
I don't think you fully comprehend how easy it is to write a new profile.

AMD used to release Crossfire Profiles which were essentially just a bunch of small MST configuration files that essentially just "Plugged in" to the drivers that gave the drivers the appropriate settings to use for the specific game and went away from there.
They ranged from about 4kb to 40kb in size for each specific game.

Even I could write one with minimal effort, it's the testing that would get time consuming... But that isn't an issue for those genuinely interested in such an endeavor, heck just think about the hundreds of hours people sink into modding or building that "one building" in Minecraft.