I'm focused on Average Damage per Attack and whether the feat is significantly better than taking instead a +1 to attribute. It isn't. It doesn't generate more significant increases than raising the attribute would, except in cases where it's not of that much importance - low AC (and generally low threat) Opponents. And the attribute generates more consistent increases in damage.
There is a certain amount of truth to what you say here. When one looks at the math for a 4th level Strength based PC with Str 16 and great weapon specialization, the DPR for +1/+1 is better at higher ACs than the DPR for -5/+10 (including the GWM extra attack for criticals).
Code:
AC, DPR +1/+1, DPR -5/+10, extra GWM DPR for downed foe (if critical did not happen)
10 10.90 12.76 0.58
11 10.28 11.64 0.53
12 9.67 10.52 0.48
13 9.05 9.40 0.43
14 8.43 8.28 0.37
15 7.82 7.16 0.32
16 7.20 6.04 0.27
17 6.58 4.92 0.22
18 5.97 3.80 0.17
19 5.35 2.68 0.12
20 4.73 1.56 0.07
When one looks at this math straight up, AC 14 is more or less the break even point. +1/+1 does better at AC 15 and higher, -5/+10 does better at AC 13 and lower. AC 14 has a very slight edge towards using +1/+1, but it really depends on the situation.
However, this is a bit misleading.
A +1 magic weapon increases the break even point to almost AC 15 (it's not exactly a full move on the chart due to the extra damage).
Bless increases the break even point to above AC 16 (i.e. better to use -5/+10 at AC 16, better to use +1/+1 at AC 17).
Advantage increases the break even point to almost AC 19.
This is at level 4 when PCs have +2 proficiency to hit. Just going to level 5 moves the break even point to AC 15.
A +1 magic weapon at level 5 means that the break even point is almost AC 16. At level 5, PCs do not fight a lot of AC 17 and higher foes, and when they do, it's not too hard to get advantage or bonuses due to spells, Feinting, conditions, etc.
And, the melee PC is not required to used the -5/+10. Comparing the +1/+1 DPR with the DPR for having GWM without the -5/+10 (i.e. +0/+0 and using the critical extra attack feature only) at level 4 yields:
Code:
AC, DPR +1/+1, DPR GWM +0/+0, extra GWM DPR for downed foe (if critical did not happen)
10 10.90 11.45 0.45
11 10.28 10.80 0.42
12 9.67 10.15 0.40
13 9.05 9.50 0.37
14 8.43 8.86 0.34
15 7.82 8.21 0.32
16 7.20 7.56 0.29
17 6.58 6.91 0.26
18 5.97 6.27 0.24
19 5.35 5.62 0.21
20 4.73 4.97 0.18
Straight up, getting the extra attack for a critical does more DPR than +1/+1. So obviously, the feat is more potent than +1/+1. Sure, +1/+1 does not use a bonus action once in a while and it gives a +1 to Athletics and Str saving throws, but this is white noise. The feat is obviously better without even using the -5/+10. And it is better at level 4/5 right out of the box. Eventually, most melee PCs will take the +1/+1 so at higher levels, a GWM PC with Str 20 is basically superior than a different feat PC with Str 20. No other feats give this level of leverage. With increases to "to hit", AC not keeping pace with proficiency, multiple attacks per round, and many more/better PC abilities/spells at higher levels, this only gets better and better.
The synergy of using the -5/+10 at optimal times and for multiple attacks per round combined with buffs is what makes the feat unbalanced. Not the straight up math comparison of +1/+1 vs. -5/+10. Your point is basically moot. It ignores the realities of when the -5/+10 will be used.