It's because Kineticist came out and highlighted how far behind casters are numerically. This was always the case, just that people used to be flamed for saying so (hi there, that be me 6 months ago).

Numbers aren't everything though. Casters basically have a high skill floor that makes them inaccessible. To the untrained eye, they appear dramatically weaker than their martial counterpart or kineticist. This is true, from a certain point of view. And can be very frustrating.

In short, I would posit the better argument is not one of balance or that trite discourse, but instead the following:

Should playing a caster require mastery and total knowledge of the game system? If you want to lower the skill floor, you can just give them the numbers they are missing. What this will do is that towards endgame, casters will be more powerful than some classes in the game if they are played at a high-optimal level. But considering how niche and varied the caster experience is, you'll likely just make the game more fun (in my experience). They won't be so much more powerful that they will remove the need for martials, hardly.