Yup and they effectively did not do that when they presented the rule in the DMG they presented a brick when it needs to have 2 others underneath to make it hold up the platform if you will. The purpose of the brick was being part of holding up a platform.
Then add the other bricks.
You have rules for flanking
You have rules for attacks of opportunity
You have rules for movement
You have rules for disengage which counters attacks of opportunity
You have rules for facing.
There is a structure here. If there is part of the structure that is not doing what you want, then start changing the structure. But, you can't look at one part that does exactly what they designed it to do and complain that nothing else exists. The other parts are there, if you want them to be more, make them more.
It sounds like your real issue is with movement and attacks of opportunity, so change those rules. Sure, it will have ripple effects, but then you just have to deal with those ripples.
Flanking does exactly what they designed it to do, give advantage to melee fighters who surround an enemy. If you want it to do something else, you need to redesign it.
Currently complaining the design goal of being modular was not really accomplished in a way that allows those who want to have even reasonable tactics to adapt it to their purpose without very large amounts of work.
But every criticism is met on here with ad populum... so I am not expecting better.
You know what though? I agree with you.
The game is not as modular as I expected it to be when they announced that design goal. I wish it was more modular.
If you want to complain that the rules aren't more modular, then complain about that. I'll agree with that, could have been more modular. But, you are complaining the rules aren't doing what they were designed to do, which I disagree with. They are doing exactly what they were designed to do, even if you think they could have done more or could have done it differently.
That to me is mixing up what flanking does with its purpose which is as part of a .collection with a purpose of making positioning more important its not really very good stand alone.
Yes, flanking in older editions was a bonus and it was also part of a larger ruleset about moving and positioning.
And it is the exact same in 5e. It is a bonus and is part of a larger ruleset dealing with movement and positioning.
But flanking alone in 3.5 or 4e didn't do anything except make it easier to hit the enemy. It was part of a larger ruleset, but those other rules were the other parts. So, flanking in this edition makes it easier to hit an enemy. If you want more, look to the other parts of the rule set.
Oh and I already acknowledged that the 5e Healing Surge did do the purpose they defined for it. They might have mentioned that if you have fewer larger battles in a day this allows one to offset the defensive impact of that.
I think that they could have even mentioned or added another variant rule for offsetting short rest characters offensive fade if you get down to regular 2 or fewer battles a day or the like.
They can't do everything man. Some of these effects you have to figure out on your own.