Another element, particular to this game, is what WoW calls "class fantasy" and I think of as concept integrity. Most of these jobs pre-exist FFXIV. Red Mage has been part of Final Fantasy since the birth of the series, Dragoon and Summoner nearly as long. They're fairly particular ideas! So it's not just a matter of "is this fun," but also is this Red Mage?
Red Mage is a resounding triumph of design because it hits all its notes. Fun rotation, looks like a red mage, feels like a red mage. Slings white and black magic, dualism is baked deep into the identity, caster with a sword, sword is important... it even has a bit of actual utility in Vercure and Verraise that make it feel like a caster-melee-healer hybrid. No wonder it's one of the most popular jobs. It's not just a good job, it's a good Red Mage.
Similarly, Dancer delivers! Its stuff feels like dancing, the rotation feels great when all the procs line up, and it satisfies the concept of being a buff-dispensing support class in a game that can't actually have pure support classes. They keep fucking with Bard because it has never felt like a Bard enough to satisfy people. The ability to play music was a great boon, but the songs have always been a vestigial part of its combat gameplay, with its archery being far more central, and the tension of maintaining songs and debuffs awkward until now. So they keep fiddling with it.
Summoner of course is the great example of concept integrity as a design driver. Was Summoner a very engaging Affliction Warlock? Sure. Do new players pick Arcanist as their starting class because they want to be an Affliction Warlock? Generally, no. They expect to become a Summoner, and are often dismayed when they discover they'll spend far more time babysitting DoTs and crunching through a busy rotation than, you know, calling out Ifrit to set their enemies on fire. So here you had a base of players that had learned to accept that the thing called Summoner was actually Miasma Mage with a Bahamut phase... but that's not ideal. It should be one of the most popular jobs. Everyone loves the idea of calling out big monsters to destroy their enemies. It's a staple of Final Fantasy. So they rebuilt it to make the summons central, and the general response has been hugely enthusiastic, even though this also made it probably the simplest job in the game. Does this suck if you loved the old Miasma Mage design? Yeah, sure-- but you're not the only person Square is thinking about, nor should you be. If there are five people who would love to play Summoner but don't for every person who is cool with what it is, well. They can put in a new DoT-babysitting caster later, but only one job can ever possibly satisfy the fantasy of being a Summoner, sooooo...