Summoner has a history of broken design. It got thunder 1 removed from cross class skills, completely broke their statistics with heavensward buff stacking. Stormblood is a masterclass in how not to design a class while also being the first time we saw the potential for a real summoner class. No, really, they took Dragoon's dragon sight tether and literally gave it to the pet, except it leashed onto the nearest ally which could be a tank, a healer, or the lowest DPS person on your team. But not yourself, and you had no real control over it. Then there's the whole 11 wyrmwaves rotation where you could cast addle to get more wyrmwaves when you were intended to explicitly get only 8 of them.

Endwalker took their pet action queue system and made it extremely easy to accidentally break it. Running the same rotation you could have the pet finish all queued actions over 5 GCDs apart. As in a rotation that ended on the 7th GCD into the fight could finish all pet actions on the 8th GCD or the 12th GCD and you had little actual control over it.

At this point players were split into 2 camps. Those who loved its warlock-esque playstyle from the early game, which didn't work well as seen with HW summoner and snapshot abuse. And those that saw the demi-summons and went: "Can this just be the class please?" The class at this point outside of pet jank was mostly a class of texture differences. How it played in DWT and DB had a different texture to post-DB, where FBT/DP had a very different feel due to rotation, and the back half also rolled differently.

The devs then took the texturing of the old rotation but simplified it a tiny bit more, most notably with FBT no longer being a 1-2-1-2-1-2-1-2 rotation and now just 1-1-1-1-1-1, took feedback of: "We like the instant casts of summoner," but forgot that summoner was still more a turret caster than a mobile caster and basically pivoted it from turret caster into rphys with some casts, and then started designing every fight like cast bars didn't exist.

A combination of all these changes led to summoner being absurdly popular. As in, I don't think any class was more popular save maybe WHM or WAR, and it dominated all of EW as the most popular class outright, and you can see why the devs are hesitant to change it. This ignores any contributing factors like casters in general being suffering in EW and the overall war being waged by encounter designers on the turret caster playstyle itself, but seeing as the devs are continuing to wage war on turret casters, there's no desire to change smn because it was the most popular DPS class outright. And it wasn't even close.

This is an extremely brief history of SMN and glosses over a lot, but we got here because of bad design decisions compounding until they hit a breaking point, and then the devs messed up when listening to feedback and also messed up encounter design. So summoner is likely the future of all casters.