If a character concept is modeled in game only by being inferior, then it's a clear message not to play the concept.
In 3e, for instance, you might want to play a Commoner, but the stats of that class make it very clear it's not suitable for an adventurer.
Yup.
In 5E, one could try to play a commoner. No matter which class the player takes, he doesn't wear armor, he doesn't cast spells, he doesn't use a shield.
He just fights with his club or dagger in combat. And is knocked out or killed a lot.
More to the point, D&D has a long tradition (and 5e has a real weakness for catering to tradition), of over-compensating specialists for real or imagined limitations or weaknesses.
The point is that the Elementalist is a totally fine concept to play in 5E without over compensating him. People seem to want more than the game system gives though.
Would an Elementalist not take Detect Magic because it is not a fire spell?
Would an Elementalist not take Identify because it is not a fire spell?
Would an Elementalist not take Dispel Magic because it is not a fire spell?
Would an Elementalist not take Telekinesis because it is not a fire spell?
Would an Elementalist not take Web because it is not a fire spell?
Would an Elementalist not take Haste because it is not a fire spell?
If a player decides to only cast fire spells and to not cast other spells that his PC took (or to take a lot of spells that have a low utility and are rarely used), that's not an indication that the basic character concept is not fine. It might indicate that the given player is a bit of a loon.
The game design is just fine.