Here is Jeremy Crawford on how they develop and design subclasses.
https://www.youtube.com/watch?v=w5SeqUY8Pjc
1. Start with the story
2. Describe the story
3. Brainstorm
mechanics to express the story
4. Compare
mechanics to other subclasses within the same class for balance
5. Ensure subclass
mechanics match the infrastructure of other subclasses. Eg. new Cunning Action for Rogues
6. Check for duplication with other subclasses <-- mechanics and story
7. Check back to ensure it is representing the story
etc.
They start with story then get to mechanics. Rather than start with mechanics then fit in the story.
The distinction is important, and important to understand in order to understand the game and
the rules.
Some people don't like it, but it's good to know it. This is 5e.
You do realize steps 3-4 are mechanics. That's 3.5 of 7 or 50% of the process. Not only that but I have been saying one informs the other... so you start with story 1&2, the you enforce mechanics 3-4, then you pulling back to story because your mechanics have informed your story just as your story has informed your mechanics. That the PHB and DMG are more than 90% mechanics. I have never said that you can't start with story. In fact, my only point is 5e is not JUST story or JUST mechanics, they are two parts of a whole and act to support each other. Your saying you can't start with mechanics and get to the same place but you actually can I have personal made characters and campaigns setting as a GM both ways. I just need a starting point and to make sure I full fill BOTH in the end. Your arguments are that that D&D is a story game and mechanics are not all that important but we have PHB and DMG that were made first and the basics of every other book for a good reason. Its a story world based on rules.... that means both. You assertion that story is king and every thing else is crap is proven wrong by the existence of suck extensive rules.
Don't believe me still?
Let's actually Quote Jeremy Crawford
http://dnd.wizards.com/articles/sage-advice/philosophy-behind-rules-and-rulings
"
Rules are a big part of what makes D&D a game, rather than simply improvised storytelling. The game’s rules are meant to help organize, and even inspire, the action of a D&D campaign. The rules are a tool, and we want our tools to be as effective as possible. No matter how good those tools might be, they need a group of players to bring them to life and a DM to guide their use."
Your link:
https://www.youtube.com/watch?v=w5SeqUY8Pjc is specific to building classes and subclasses but ok, lets look at it.
What is the story
0:44 - "Every design... at least the best designs. They start with a story" --So he caught himself mid sentence to correct that not all designs start with a story and "best" is subjective. He goes on about this being a mission statement starting point. I have played characters based on a mechanical role in a party and so have many other players... for example... when asked to join a Champaign I said sure, the party then said, "Hay we need a scout, do you mind playing a scout of some type?" This was also a mission statement but not based on a character story but mechanical desire so I used that to inform my story playing a warlock scout who came from the village of the party leader then tied him to the role of scout by giving him devils sight and I made him a surviving street urchin (for scout skills) who's family was killed by the same band of cultist that killed the party leader's family. This created a story bond to a party member and a story reason for me to have evolved as a scout as having a mechanically superior "darkvision" which leads to my character applying himself as scout to be useful in a group by making use of a mechanical advantage... so he is not just saying he is a better scout than the human paladin … he is actually better at scouting and so naturally fills the role due to having this above average ability and a desire to find some way to help. That is not me disagreeing with his approach. I am just saying their are 2 and he has a favorite.
2:46 - "Sometimes we will end up making changes to the mechanics that push the story in a new direction. And then we go back ...and sort of talking on the practical side. We go back and actually change that story text so that it now reflects the new story that the subclass is telling" ...So Jeremy Crawford in the link your using to discredit my statements said that … sometimes the mechanic changes the story.... Story informing mechanics...Mechanics informing story... just like a said the whole time.
3:15 (Talking about Mike Mearl's process, don't want to have to word for word the video your welcome double check me) "where you get to see some of that early concepting phase of lets spit ball something, lets come up with a story, and then lets brain storm some of the little class features and other
mechanical nuggets that could express that story. And it really is a way you just let your imagination run wild. ...With some limits. One of the big limits even in that initial phase is that your playing in kind of playground that already exists"
...that is him saying you can't make the story without considering the rules as a container "playground" he goes on to talk about the "mechanical heft" of subclasses and keeping the mechanical weight of a subclass in comparison to its primary class in mind not to balance classes but to keep a mechanical standard of priority and power of subclass features vs class features... meaning that story being a starting point does not mean your not constantly looking at mechanical design building story in a mechanical vacuum.
You need both. This idea that D&D is a story game only or a Mechanical Game only is frankly against reality. It must have both to function. They are both equally important no matter which one you start with.