The bonus does at least theoretically equate to a more powerful being, because while a level-appropriate enemy will have the same bonuses and thus be on the same power level (how well balanced that power level is is its own debate that is already raging, so setting that aside for the moment) other creatures that were once a threat now... aren't, really.Yes, +Level is good at tightening up threat ranges, and, also, 5th Ed may have gone a bit too far with the BA thing.Why do some people want the threat range tightened up?
If people want to fight new kinds of monsters, ones more powerful than before, won't a fast experience point progression work just as well? Instead of 1,000 xp to the next level, make the threshold 500 xp.
I'm speculating, as I don't think I've seen it stated explicitly, but it seems to me that the "zero to hero" model of D&D/PF is important to many in terms of the feel of the game. I suspect that beginning out as a supertalented farmhand and rapidly becoming a mover of mountains and shaper of world events is a key part of what makes Pathfinder Pathfinder to them.
As such, still being threatened by the mooks that used to push you around when you started detracts from the "power to rival demigods" feel of later levels. When you've got a built in +12 or something, it's much easier to laugh off any but the most significant horde.
It was certainly a criticism I heard levelled against 5E (who had as one of their goals to retain the threat of lower level creatures in sufficient numbers, even to high level PCs). People who didn't like that approach sometimes labelled it "not D&D enough" - as if the goblin-orc-ogre-giant-... progression was a key element of what they were looking for.
I am in favor of the built-in +12 at 20th level. But I feel that the built-in +20 at 20th level is crowding out the interesting feats and features because the numbers are too good. In Pathfinder 1st Edition, we have the running gag that martials can't have good things. The motivation for the gag is that the full BAB at +1 per level is a gigantic numerical bonus and does not leave much room for other good things. I fear that Pathfinder 2nd Edition, which gives every class +1/level, will extend the can't-have-good-things to every class. The class feats and skill feats and nerfed spells offered by PF2 do seem lackluster.
The progression in Pathfinder 1st Edition says that a character is supposed to become 41% more powerful at a level-up. In Pathfinder 2nd Edition, the progression modeled in Table 4 on page 21 of the Bestiary uses the same 41% improvement. With a 60% chance to hit an opponent, a +1 to attack rolls would be an 16.7% improvement, because both number of hits and number of critical hits improve. A +1 to AC would be a similar improvement. A boost to hit points is also an improvement, and all three already add up to over 41%. My rough estimate from a few examples is that in Pathfinder 2nd Edition, a level-up is really a 68% improvement in combat prowess.
And monsters improve at the same rate. Thus, in Pathfinder 1st Edition, a monster two levels above the party's level would be twice as powerful as a party member and three levels would 2.8 times as powerful, in Pathfinder 2nd Edition, a monster two levels above the party's level would be 2.8 times as powerful as a party member and three levels would be 4.7 times as powerful. Monsters below the party level would be proportionally weaker, too. This tightens the threat range, so that instead for fighting monsters from 4 levels below and 3 levels above, the party will be restricted to fighting monsters from 3 levels below and 2 levels above.
Skill challenges will work the same way. Instead of the party going from climbing a wooden pallisade of a goblin village to the stone-block wall of a castle, the party will have to go from that wooden pallisade to an featureless stone wall. It won't be as extreme as the combat situation, because usually in PF1 one skill monkey in the party kept up the +1/level in any one skill, but when everyone has that skill level, the design criteria change.
I was wondering whether some players really want the challenges to increase at such a high rate by level, where the party cannot win against a creature three levels above them and the rivers in mid-level adventures all have white-water rabids and slippery moss-covered broken bridges. That would be a solution to the problem of the numerical bonuses being too good for a 41% improvement: toss out the 41% standard and go with a new 68% standard.