Have you really noticed these things, or are you just guessing by looking at what you're reading? Because in actual game play, I'm not noticing that scenario at all. I guess it comes down to who you play with.
It's certainly where we noticed it start to break down. It's the point where attack bonuses start to go off the bottom of the die. Where you first get +9, +10, and +11 to hit.
Look at it like this:
At level 1, you have +2 Proficiency, +3 Ability for a total of +5.
At level 9, you have +4 Proficiency, +5 Ability for a total of +9.
At level 20, you have +6 Proficiency, +5 Ability for a total of +11.
Levels 1-10, you get +4. Levels 11-20, you get +2. You get the lion's share of your attack bonus from 1-10.
Additionally, IMX (including official modules), you start to find +1 weapons about level 4-7, +2 weapons about level 8-12, and +3 weapons about level 13-16. Yeah, yeah, "magic items are not assumed blah blah blah." Show me a level 10 Fighter with a mundane weapon and I'll show you a player who thinks he or she is getting the short end. Magic items aren't assumed in design, but magic items happen in real games and it's one of the most common rewards in nearly every campaign.
Thus, in the games that I've played, most 10th level characters have an attack bonus of +10 or +11. The
maximum attack bonus in the game is +14 (without some ability or effect that lets you break 20 ability score, +6 proficiency, or a +3 weapon bonus). When you're at +11, you hit AC 13 on a 2. You hit AC 18 on a
7. Once you get to about level 10, characters just hit all the time on every attack. That's what they do.
On an Archer with Sharpshooter, you're looking at a base ranged attack bonus of +11 by level 9, but more likely you've got an attack bonus of +12 from a +1 weapon. With a +12, you hit AC 14 and below on a 2, and you can either deal 1d8+6 95% of the time (9.975 avg damage) or 1d8+16 70% of the time (14.35 avg damage). Against AC 18, you're looking at regular damage 75% of the time (7.875 avg) or Sharpshooter damage 50% of the time (10.25 avg). You know what the breaking point is where you should stop using Sharpshooter?
AC 22. You know how many monsters in the Monster Manual have an AC of 22 or higher?
8. There's six Ancient Dragons (black, blue, bronze, gold, red, silver), the Empyrean, and the Tarrasque (the only one above AC 22). It doesn't matter if you've got a
+1 heavy crossbow, either. The breaking point is still AC 22. If you couldn't optimize your stats and only have a +4, or if you somehow never got a +1 weapon, then the breaking point is instead AC 21 (which adds green and copper Ancient Dragons, as well as Solars).
So, at level 9 Archer with Archery style, the Sharpshooter feat, and either optimized stats or a magic weapon should use -5/+10 on essentially everything in every combat. That's the mathematically correct option.
Now, if you've got some non-weapon damage to add in, you might want to attack without Sharpshooter -- as far as I recall the feat says ,"before you attack," and not, "before you take the Attack action" -- so you can freely switch between regular and Sharpshooter as needs be.
This is why I would rather Sharpshooter said something like, "Black Arrow: Your ranged weapon attacks ignore all types of damage resistance," or, "Sniper: Your ranged weapon attacks have an extreme range category that extends to double the weapon's normal maximum range. You have disadvantage against targets at extreme range." Either option would be preferable.
GWM is actually less abusive than Sharpshooter, because there's no attack bonus equivalent for Archery style, because heavy weapon style increases the weapon's damage (2d6 goes from 7 to 8.33), and because heavy weapons deal more damage than ranged weapons do in general. That throws the math back towards not using -5/+10 a bit more. Still, I'd rather that feat said, "You deal 1d6 additional damage to creatures one or more size categories larger than you." I miss two-handed swords dealing 3d6 against Ogres.
