Roger is correct. D76 1+1 is well over half strength when developing is complete.
I only wanted to note that some developers (HC-110, Rodinal, various Pyro formulas) may also used used in low concentrations which are exhausted when developing is complete. This would be for stand developing, of course.
The main benefit to D76 1+1 is that you throw it away when you are done with the tank load, and save half the volume of D76. (D76 1+3 is often used for large tanks since they often have so much wasted volume).
Even then, I suspect that in some cases it's oxidation that kills them off. Of course that's a purely academic point, because you don't really care why a developer stops working.
It is facinating to reflect on how little developing agent is required. As an extreme example, consider a 2-bath developer in which the first bath is 5 g/litre of metol and 100 g/litre of anhydrous sodium sulphite to act as a preservative.
Development in such a bath takes 15-20 minutes, but if you use it as a fore-bath for 2-4 minutes, tip it out, and tip in Part 2, 6 g/litre of anhydrous carbonate and 100 g/litre of anhydrous sodium sulphite, development will be complete in another 3 minutes or so.
In other words, all the developing agent you have is what soaks into the emulsion in 3 minutes, plus whatever wets the film, reel(s) and tank: maybe 25 ml for a 450 ml two-reel stainless tank, or 0.0625 g of metol per film.
It's been decades since this was my habitual developer, but I'm pretty sure that one of its attractions was that each bath kept half way to forever and could be re-used indefinitely. Much like Diafine, really, though the subject of fewer exaggerated claims.
Cheers,
R.
