Well, you'll likely only get rid of the old stuff in theory anyways. All the vendors will need to provide drivers for older versions of OpenGL, otherwise people will be pissed that their favorite games and apps stopped working. They will most likely provide these in the same driver as the new stuff. They will most likely yet again introduce an official "compatibility" extension that implements all the old functionality and is optional in theory, but in practice will be implemented by all, since they have to ship it with their driver anyways.
Except there is no reason for that to be the case.
OpenGL up to 4.5 and beyond using the old model can live in it's own code path, as it currently does (note the separate GL dlls which come with your driver - it is already split up) and in it's current state could live on forever, heck if they wanted to layer it on top of GL:TNG they can as long as the functionality remains the same.
GL:TNG however can be, and will be, a new software stack. There is no need for any of the old functionalty/extensions/programming model to exist because it simply won't be there or needed. This is a DX9 to DX10 breaking change; the API changes, the rules change AND with NO EXISTING SOFTWARE using this API they are free to do what they want.
And that has been GL's problem ever since OpenGL3.0's failure to redesign the API; the API remained the same and so people using it would just layer on top of it which is why you couldn't remove the functionality. Heck, there was no push to remove it because it would be so entwined in the existing code base it would probably be more work to pull it out and separate into a new 'new only' code path that keeping it makes more sense.
GL:TNG doesn't have that problem.
No one is using it.
There is no compatibility to maintain.
This is Version 1.0 of a brand new API, folded under the banner of OpenGL but as distinct as D3D9 is to D3D11.
And with all that stuff removed,
Without the need to deal with multiple extensions influencing the state,
With a more explicate model where the application drives the functionality,
Without all that the driver itself becomes simpler, does less work, has less overhead and is faster.
OpenGL4.5 won't suddenly go away and stop working, it'll stick around for a while in the same way that D3D9 based games still work but if you are coding OpenGL4.5 then you won't interact with this new model at all because it is a different API.
So there is no worry about interactions, no worry about compatibility layers and other things gumming up the GL:TNG because it isn't needed.
You might think 'well, its dumb to throw all that compatibility away' but the truth is professional developers are use to working with multiple APIs, we've done a D3D9 to D3D11 transition, we've dropped the PS3 rendering code in favour of the new API for the PS4, teams right now have, or are, jumping on Mantle and implementing it in a matter of months. We deal with multiple incarnations of the GL|ES runtime. Apple's Metal is a welcomed thing in the industry.
You might say 'what about the hobbyists?' and to that I say 'what about them?'.
OpenGL is not "the people's API" - if it wants to stay relevant it needs to be driven by the needs of the big AAA studios (who are the ones who sell gfx cards too).
Hobbyists still get OpenGL as it stands now.
Those wanting to enter the industry will be wanting to use the newest API.
And those not wanting to care will pick an engine like UE4.
And this is the best future.
It's a way of working we are use to.
It's a way of working which will get the best results.
It's a way that MS has successfully used for a few versions now.
Compatibility with 'now' isn't a problem.
And this is a
good thing.
