The first question is what exactly do ‘carbs’ do for beer? The answer is they add ‘mouth feel’, ‘body’, and in some cases flavour(s). They may also assist in head retention. The second question is where does beer get the carbs to do these things? The answer is non-fermentable carbohydrates in malted grain for the most part and sometimes non-fermentable carbohydrates in other stuff added to the malted grain to produce specific beer flavours or types. Hops, for example.
‘Mouth feel’ and ‘body’ are the reasons beer is often called a ‘barley sandwich’. While 95% of any good beer is just water, a mouthful of beer tastes a lot different from a mouthful of water because it feels a lot different. And that difference is not simply the ethanol. The physical sensations caused by a mouthful of good beer make it feel much ‘thicker’, ‘richer’ and ‘heavier’ than water. This even though the solution of ethanol makes beer less dense than water!
Can we reproduce this ‘sensation of beer density’ without recourse to added carbs? I think the answer is ‘Yes!’ I think I have already demonstrated this in my experiments noted above.
Now I agree that it is unlikely any commercial brewery is going to take the risk of producing a zero carb beer similar to what I’ve been doing. But that doesn’t mean it can’t be done. Done well and result in a class beer comparable to any quality beer you can name.