Why does everyone keep saying this? No one is suggesting using the optional flanking rule and not allowing use of the Help action, so it's a bit nonsensical to keep presenting them as alternatives. I guess this is just the way people who don't like the optional flanking rule are choosing to say, "Don't use the optional flanking rule. Just have flanking creatures use the Help action." I find this unsatisfactory because Help isn't generally applicable to situations in which flanking is occurring and is only advantageous if there is a significant asymmetry in the power level of the flanking creatures. In any situation where one creature isn't significantly more powerful it's actually a very bad idea.
First of all: Don't care so much about min-maxing. It shouldn't be about highest damage output, but more about what leads to the most fun. Help really doesn't need to increase the damage, as it makes combat more interesting, less damage is fine too.
Second: The rest is all about balancing so that combat is more fun to play. For this, you should first realize that even with just normal attacks, when you are fighting X enemies, it's usually the best strategy to gank up on one, so there are less enemies left that can damage you on their turn. Spreading up attacks on the other hand is already rarely useful. If you add the flanking rule to this now, then focusing on a single enemy gets even better. So in the end combat will lose a lot of strategical decision because all on one enemy will be what everyone does.
Help is a substitute in so far, that it allows you to narrate something as flanking without actually making the attack even better. Help is a lot more interesting because in many situations it's worse but in some situations it's actually better and there are nice synergies with other skills (buffs etc.). Using help as substitute for flanking makes combat consequently much more balanced and interesting.
That's why I'm saying it's a good substitute for flanking.
It's a standard rule that the DM can assign advantage to a roll for any circumstance the DM decides will have a significant effect on the roll's outcome. This can include situations in combat such as being flanked by opponents or opponents holding the higher ground. The optional flanking rule is merely a suggestion as to one way in which the DM can exercise his role as the granter of advantage and disadvantage and do it in a way that's consistent. This is a basic function of the DM in the standard rules, so the adoption of optional flanking doesn't change the way the standard rules function one iota. The DM in one of your standard games always has the ability to grant advantage and doesn't need an optional rule to do it for flanking.
You see the good thing about being free to give advantage as DM rather than binding it to a fixed situation is, that I can grant it sometimes and sometimes not. It allows me to reward good narration. I could decide to not give advantage when someone just says "I attack X from the side", but decide to grant advantage for a really clever situational idea the player comes up with to gain advantage on the next attack.
Using the optional flanking rules takes away from that freedom. You suddenly have to almost always give advantage to attacks. You can no longer award good ideas very well and in the long run, it will make combat more boring, because players won't feel motivated to come up with clever ideas.