I forgot to do the math on the staff if you were to give it 18-20 natural range. It would go from 997.5 in 20 attacks to 1282.5, still hardly competitive but better.You're comparing apples to oranges. Its hardly fair to compare preu37 loot with post u37 loot. There was a complete overall on the dice of endgame gear whereas 1d6 vs 1d6+6. Lets take the Gulthias Staff for example. This weapon is 5[1d6+6] and it isn't meant to mimic a weapon that has extremely high base damage. I understand that that is still higher than say the rapier from u37 which is 1d6+3, or even the khopesh which is 1d8+3, however, when factoring in crit profile both of them outperform.
Lets use your math but lets factor in feats since the crit range increase is different.
"A Staff of Shadows with 5[1d6+6] and a 19-20/x3 does (5 * 9.5 = 47.5) on a non-crit and (47.5 * 3 = 142.5) on a crit. On an average 20 hits, they will miss once (0), hit 18 times, and crit once. 18 hits (18 * 47.5 = 855) and 2 crit (2 * 285 = 142.5) make a total of 997.5 damage across 20 hits, for an average of 49.875 per hit."
Lets apply that same math to the khopesh or something with a similar to the crit profile of the ESOS that you guys are mimicking.
Calamity, with 5[1d6+3] and a 17-20x3 does (5 * 6.5 = 32.5) on a non-crit and (32.5 * 3 = 97.5) on a crit. So out of 20 hits you will have 3 crits on average and 16 hits. 16 hits (15 * 32.5 = 520) and 3 crits is (4 * 97.5 = 390) now lets take into consideration that most, if not all, of the other weapons have on hit damage. So using calamity 2d8 and crit damage that it has your looking at another 10 damage per hit and and average of 239.5 damage per crit which brings the numbers to (190+ 239.5 * 4 = 1,148). Taking this and with the base damage your looking at (1,148 + 520 + 390 = 2058) This isn't taking into consideration the disintegrate.
Im aware there are other weapons, most of which have at least 9d6 extra damage per hit, so that would be 31.5 damage per hit, or in other words 598.5 damage out of 20 attacks.
The staff doesn't compete.