Can you help me out with your math? I think I did not understand it. Let's say I have a D of 10 and H is 0.5. This results in a ratio of 20. If I take GWM and use it, D goes to 20 but H drops to 0.25, right? And in both cases, average damage would be 5. But taking the ABI would raise D to 11 and H to 0.55, also raising average damage to 6.05, and the conclusion I get is not aligned to yours.
Actually, by fixing D at 10 and varying H from 0.3 to 0.95 (so to avoid saturation) and checking the results for which GWM or ABI would be better, then fixing H at 0.75 and varying D from 5 to 20 and doing the same check, I was not able to define a single ratio to be the break even. In the first case, the tie would occur at a ratio around 16, while in the latter, it was 14.
Algebraically (?) He is correct - if you take the comparison between the two values of damage per Short Rest of D*A*R*C*H (where A=Attack per Round, D=Damage, R=Rounds per Combat, C=Combat per Short Rest, and H=Chance to Hit) and compare GWM vs +1 ASI - you get the following equation
GWM is Better......
Where D*A*R*C*H (GWM) > D*A*R*C*H(+1 ASI)
[A, R, & C are equal on both sides so remove and add modifiers for GWM and +1 ASI]
Where (D+10)*(H-0.25) > (D+1)*(H+0.05)
[Simplify]
Where (D+9)*(H-0.3) > D*H
[Simplify]
Where DH+9H-0.3D-0.27 > DH
[Subtract DH and Add 0.27 and Times 100]
Where 900H-30D > 27
[Divide Both Side by 30 And Round up, because ain't nobody got time for 27/30]
Where 30H-D>0
[Add D to both sides]
Where 30H>D
or to complete the full statement
GWM is better where your Damage per attack is 30 times greater than Your Chance to Hit (30H>D)
This of course assumes no advantage, trivial or impossible to hit requirements (Due to extremely high/low AC), and using GWM at all times