These arguments are difficult without context. If the rogue is routinely getting sneak attack damage then that is an average of +35 to damage.
To get your number, the creature has an AC in the range of 39 to 41. How does this stack up? There are 4 monsters in the PF SRD at CR 20:
Balor AC 36
Ancient Gold Dragon AC 39
Pit Fiend AC 38
Tarn Linnorm AC 36
Of course, a 20th level rogue should have a +5 weapon (now needs a 13 to hit the Gold Dragon), has master strike, and flanking gives a +2 bonus (and she should be able to arrange that about 75% of the time with tumbles). Sneak attack damage cuts through DR if the party doesn't have the right weapon. Tumbling makes flanking easy.
I am not saying that the skill based class is better than the combat class but you seem to dismiss a lot of the abilities of the rogue.
Now this can happen for several reasons. If nobody knows how to construct or play a rogue it may feel suboptimal. House rules (the little variations we all make to the game) can matter too.
For example, I had a DM who interpreted sneak attack as needing to be set up like a 2E backstab (only woks from behind on a completely unaware opponent). Needless to say, that game saw no pure class rogues.
But I do think that the class, as per the rules as written, is better than you are giving it credit for.
Does your DM pull stuff straight out of the book? Maybe most DMs.
Our DM designs our encounters around the capabilities of the characters. Which means he uses the BAB and abilities of our toughest melee fighter as his barometer for the enemy's AC. For example, if we were fighting that Ancient Gold dragon it would cast
Mage Armor and
Shield boosting its AC to 47. He would give the Balor armor or Bracers of armor +6 and a
ring of protection +4 boosting its AC to 46. Possibly toss in an evil priest to buff it.
I assumed this is how other DMs designed encounters so their players don't walk over an encounter. That's why I listed only BABs. Creature doesn't matter very much if the DM is designing encounters based on the highest BAB and damage capacity in the group. And the +20 BAB classes are often going to have a much higher BAB and be able to boost it even higher than the rogue.
They get the same flanking bonus as the rogue.
No. I don't ignore all the rogues other abilities. And they occasionally come up when the DM wants to let the rogue shine a little. But most modules aren't written to make a rogue shine. They don't include a series of encounters that require rogue skills, just traps now and then.
And I don't even know if a rogue would make it a dragon with that high will save against fear. That would offset his flanking bonus right there. Then the balor aura would burn him up and the gods help the rogue if the Balor decides to implode him. He's target number one along with the arcane caster and both are probably going to go boom.
Over the years playing 3E and now
Pathfinder the Rogue has had the same problem: you can get the most bang out of a multilclass rogue. Single class rogue leaves you with too many commonly exploited weaknesses such as fear, hold, death spells, and spells that do damage based on fort saves like
Horrid Wilting. And it creates problems for the DM designing encounters to challenge his characters. If he makes a creature tough for a +20 BAB class to hit, its going to be nearly impossible for a rogue to hit.
I think they should have lowered sneak attack damage and boosted BAB for the rogue, at least when doing what they do best.
Have you experienced how much
Pathfinder powered up the archer?
Have you experienced the joy of being a DM when a paladin smites evil?
Have you seen a fighter weilding two medium sized weapons without needing feats because of weapon training?
Alot of classes got boosted hit abilities except the rogue. Makes encounter design difficult. If you design a class low enough for the rogue to get a few hits on, the +20 BAB classes are going to have a field day. If you design an encounter for a +20 BAB class to have problems hitting, the rogue is going to rarely hit.
Better to take less sneak attack and hit more often by multiclassing. It's no big deal. Our group can continue multiclassing. But I would sure love to see a straight class rogue that was playable to 20 without making encounter design hard or making a player feel useless in the most important encounters in an adventure.