The issue boils down to a few things that people arent actually giving any thought to. They are:
1) All classes are balanced in relation to one another. Every single one. There is no exception to this.
2) This means that all player skills, broadly speaking, are limited to a certain scope of potential. Whether that is damage output, Status, Range/Size, resource cost, etc. There may be some variance between teh skills, but they still fit within a certain "range".
3) All classes, as a result, must be within range of each other power wise. While you could have differences in power output, those differences will only show up broadly at the highest level of play under certain conditions. Even then, the differences in power are not so drastic that one class is inherently a must pick when it comes to balance proper.
Because of these 3 factors, the defining differences between classes is NOT the skills in of themselves,because when you boil the skills down to their very core, theyre all within relative range of power to one another, but the play styles of the classes themselves. Plenty of you have pointed this out. As an example, Unleash and Overpower are relatively similar. The difference is slight in power and aoe shape when you boil both down, but both skills are comparable. One may be slightly superior, but that doesnt mean that DRK is by far better than WAR. They have ups and downs in certain regards.
To help illustrate this point, imagine every skill is broken into a 30 point system that can be divided up into 3 categories: Damage, Utility, Range.
Lets say:
Monk Skill A has a breakdown of D:15, U:0, R:15.
Dragoon skill A has a breakdown of D:20, U:5, R:5.
While both skills are not identical, their maximum values overall is the same. The power is just distributed differently, but theyre balanced in relation to one another. Neither skill exceeds 30 points of overall power. Monks skill might be better than DRGs, depending the situation, but neither has a skill that is so drastically superior that it is better in all situations by a noticeable margin.
Because of this balancing breakdown, play style affects really how much bang you get in the end. Thats what really separates the classes. Some classes may lean towards more utility than DPS, or more range than Utility, or whatever you want, but theyre all viable. Those playstyles become the identity of the class.
NOW, how does BLU fit into this?
The trouble with BLU is that their skills arent designed for PC use initially. BLU skills dont come from NPCs with players in mind. They come from Monsters who are designed to kill player groups. What this means is that Monster Skills do not fall within the scope of the normal limitations of player skills. So in my hypothetical example above, player skills may fall within a range of 30 points, but monster skills may fall in a range of 90 Points by comparison since it is designed to kill groups of players. This discrepancy lies at the heart of BLU. It would be impossible to balance the game if BLU learned monster skills as is and those skills werent re-balanced to other classes. They would be flat out OP in all content.
Everyone who's a proponent of making BLU part of the usual roster all seem to agree on that factor, and do not mind re-balancing. This is where the disconnect happens.
The playstyle of a class that has such high powered skills and can solo content is vastly different than one that cannot. In the case of the Former, the playstyle would not likely revolve around rotations, priority systems, or resource management as every other class does. Instead it would rely more heavily on picking the right skill for the right situation. The playstyle is more meta rather than more reflexive. It takes a different player skill set to think about how to handle a situation vs learning and memorizing rotations. This is supported by the fact that there is no "hard" skill path. You can learn any skill at any time (supposedly) unlike other classes, so having a larger more expanded kit to pick and choose how to handle content. Also, since you can learn anything at any time, the likely hood of rotation systems or priority systems is severely limited.
However, if you dumb the skills down, youre no longer in the situation where you can solo content and now are more geared for party play. This means the aforementioned system of picking skills for the situation gets thrown out. Instead, they would need to implement a new playstyle. One that is either rotation/priority or resource management based. This then becomes the defining identity and factor of the class. When this happens, it becomes just like every other class in that regards. You end up homogenizing it in efforts to balance it. The whole idea of learning monster skills also becomes moot because at that point, as learning skills from monsters is just a harder method of getting skills compared to other classes with no better yield because the skills have to be balanced to be as powerful as any other class. Because skill acquisition is RNG, not done in any order, and actually by-passable, this complicates how to balance the class at lower levels, unless you mandate that they need to acquire certain skills, which means it becomes just like any other class, except hard to get skills again.
The initial identity of BLU revolves around using MONSTER skills with a playstyle that fits into that theme. HOWEVER, after nerfing things, the identity of BLU switches to whatever the new Playstyle they generate where acquiring skills from monsters is just a side objective. In this regard, BLU loses its unique identity and becomes "just another caster class" with skills that are no better or worse than any other caster broadly. This is what is meant by "Re-skin", or by saying "You just want a caster thats BLU in name only."
This is the issue Im not seeing people resolve. Once you nerf the skills, it doesnt have the same identity as it initially did. It's new party playstyle, whatever that may be, becomes the new identity and what differentiates it from other casters. That new identity will have to fall within a specific range of power that is similar to all the other casters. Where learning monster skills is completely moot because power wise, its no different than any other class. So the gimmick is only novel in as far as the method of getting your skills, but not in application. The skill application is no different than any other casters in that regards. Instead of getting skills from an NPC every 5 or 2 levels, you just go grab them from a monster with a bit of RNG. But it would ultimately end up being just another caster.
It wont be BLU anymore, at least no in spirit. It just becomes a caster with a new rotation/priority/resource management system that has a harder time acquiring its skills compared to other casters. Thats what it boils down to. So when people say things like "Make it work no matter what," those people are sidelining the core aspect of a BLU mage. This really then is just saying "All I really wanted was a new caster class thats called BLU mage."