That strikers me as an odd claim. I'm having trouble seeing what else the PC build mechanics are for! They don't have any intrinsic value that I can see.
I should have put the phrases "all of their" and "in the core rules" in that sentence.
Players can play their PCs anyway they want, but they shouldn't rely on the game design to cater mechanically to all of their roleplaying / character creation preferences in the core rules.
There are standard classes and then there are corner case classes (like in my mind, elementalists).
In 3E, the vast majority of corner case classes were called prestige classes and were not in the core books (even though some prestige classes were in the core books).
This entire discussion of a pyromancer closely relates to a 3E Elemental Savant which came out in the Tome and Blood (which was about the 4th 3E splat book to come out) and later on in the 3.5 Complete Arcane book, again not a core book. I suspect that the same thing will happen in 5E.
An Elemental Savant is not in the 3.5 SRD because it did not show up in any of the 3E/3.5 core books like the Arcane Trickster (a subclass that again shows up in 5E core).
So my point is that core cannot have everything in it that players want to play.
I have never had anyone want to play an elementalist in any of my games over the decades. But earlier this year, I had a player who was not familiar with 4E want to play a thief who can cast a few minor spells. In 5E terms, he was talking about an Arcane Trickster. My anecdotal experiences do not mean anything, but the designers of 5E did not think it was important to put an elementalist in the core rules (and I personally have never played with anyone who wanted to do that). So to me, an elementalist is an obscure corner case concept/class.
I suspect they put Arcane Trickster into 5E because it was a core prestige class in 3E.
They did put one elementalist feat into 5E, but again, I think that the reason to do that was not so that players could design elementalists. I think that the feat was there solely for a mechanical reason (to overcome resist) and nothing more. Feat design has often been more about "how to overcome a certain mechanical effect" than anything else.
If there is a monster defense in the game that the PCs cannot directly overcome, then some game designer is going to come up with a feat to do it. Monsters have resist, feat. Monsters have immune, feat. Monsters have insubstantial, feat
It's inevitable, but it's not necessarily in core. In the case of Elemental Adept, the feat to overcome the monster defense is in core.
But I can pretty much guarantee that eventually, there will be a feat, or subclass, or spell to directly overcome nearly every monster defense and special monster attack out there.
