Doubling would be game breaking. Stacking a +1 magic weapon with a +3 inherent bonus in mid-paragon would not.
Adding a +1 over the expected bonus had about the same impact whether you were at 1st level or 21st. It was one of the more depressing things about 4e, thus 'treadmill.' With the treadmill, even a few stray +1s could skew things, and there were always one or two out there however much errata kept batting them down. Adding one, maybe with some kind of simple house rule like "If you have both an inherent and an enhancement bonus, increase the higher by one," could be iffy.
You received magic items earlier in the game than when you got inherent bonuses.
With standard wealth/level assumptions. Presumably not so much in a low-magic game.
And you wouldn't have the magic item treadmill of 3e/4e where you dump favourite magic items (such as a flaming sword) on a regular basis.
If you'd used inherent bonuses, you wouldn't have that phenomenon, yes. When your inherent bonus exceeded the enhancement bonus of your flaming sword, it'd still be able to inflict plenty of fire damage, for instance. You could 'drop' magic items quite infrequently, and whether they were a slightly higher bonus than the party's current inherent bonuses or not, they'd be significant.
As you imply, there's the kindasorta solution of just increasing the bonuses of the magic item to always be above the inherent bonuses.
Not what I implied at all. The only 'solution' you'd need would be to avoid placing any of the exactly 3 magic items that give nothing but an enhancement bonus, as they'd eventually become superfluous. All other items would have remained useful for their properties and powers.
Now, you would have to throw out make/buy of magic items, but that seems like it'd be standard practice for a low-magic game, anyway.
But that was a wonky and rather awkward, requiring constant bookkeeping.
No bookkeeping required, you use the higher bonus, like any other stacking rule.
Of course it's not true. That was the point of the obvious hyperbolic argument.
And if a fighter doesn't have that great attack, then using his Action Surge for other actions is an alternative.
A wizard, whether he has a nice attack spell or not, has many other useful, some even unique, spells he can cast with that same slot. A fighter's class features greatly enhance his attacks - whether the player willfully builds against those features to make his attacks poorer in spite of them or builds his character sensibly, the features are still there, making his attacks better than they would be if he weren't a fighter - but have little or no effect on other actions he might take. A fighter's help action is prettymuch like anyone else's help action. His attack action is improved by features like combat style, good weapon proficiencies, extra attack, extra ASIs, improved crit or CS dice. Using Action Surge on the former is probably a waste - it should be pretty rare that it's terribly effective to use help on two different allies - while the potential of doubling-down on the only action his class features make him significantly better in some subsequent round is worth reserving the Action Surge for.
Even with Action Surge a fighter can only use one maneuver each turn. An extra action doesn't help for that. And if the fighter uses AS to get an extra maneuver, they're not necessarily dealing extra damage.
The majority of maneuver do more damage via the CS die. The stand-out exception, Commander's Strike, grants an ally an attack - presumably in the hopes it'll do yet more damage.
Why does it matter if the fighter doesn't have a class feature that makes Help better?
Because he has features that make his attacks /much/ better. Use help when there's no point to doing anything else, and no need to use Action Surge to slip in that help action. Let someone else who has no effective actions to perform use Help. It's a third-string action. Reserve Action Surge to double down on actions you're actually better at, that combine to achieve something. For the Fighter, that's attacking, because there's nothing else he's that good at, and because another set of attacks on the same target is stacking by definition.
Advantage to an ally is advantage. It's still potentially a higher DPR boost to the party than the fighter wiffing a couple times.
You're assuming a fighter whose worst at fighting in his party, when the point of the class is 'best at fighting.'
(Plus, all it takes is a single maneuver that boosts the Help action. Super simple to make.)
Maneuvers are used as attacks, so once you hit 5th, you'd never need to waste an Action Surge to Help, you'd just make a set of attacks and use the Hypothetical Helpier Maneuver - much more efficient.
It's not sub-optimal. It's just sub-optimal at damage.
Which is the only thing the fighter's abilities make him significantly better at. Optimizing something you're bad at isn't optimizing. Intentionally sabotaging something you're good at when you gain nothing from the sacrifice, well, I don't think we have any pithy jargon for what a bad idea that is.
Unless you're advocating that taking actions other inflicting damage is suboptimal.
Not at all. It's just so for the fighter & Action Surge, and only because it lacks class features that do anything else that takes an action all that well. The idea that damage is the only thing that matters is born of short-sighted white-room 'optimization' that is, well, sub-optimal in itself. ;P Let's not go there, it is a silly place.
And since the bar for balance is so low, it should be easy for an unofficial product to be "close enough" to balanced. Those products should be viable alternatives.
There's more to quality than balance, but, yes, the onus is on the DM to make material work, official or otherwise. None-the-less, DMing is tough enough that I can understand some being leery of taking on any more such burden than strictly necessary.
