If BLM does the same damage as SAM it would absolutely break balance, there is only so much loss of DPS the general playerbase will tolerate before the utility becomes meaningless.

Going right back to the core, fights are designed with a DPS split of 2 Melee, 1 Caster and 1 P-ranged.

Each DPS role is designed to do a specific amount of damage compared to the other roles. Melee > Caster > P-ranged. This stems from the perceived difficulty in keeping uptime. [NOTE: I know this isn't the case with more recent fights and this is a problem with fight design in general. If this is the direction they want to go in for the future, then all jobs need to be looked at.] The harder it is to keep 100% uptime, the more damage the job is designed to do, which also meant you lost more damage when you couldn't hit something, which helped to balance things out more.

Now, using this model, if we have a BLM who can do the same or more damage than a SAM, where the BLM can have a higher uptime, this breaks the optimisation the SAM has to do in order to try and stay in melee as much as possible. BLM is now the easier job to keep uptime AND it does more damage, no contest, it is the better job to take.

If you take this into the context of party composition, BLM has basically become a melee, which means you end up with an effective party of 3 Melee and 1 P-range. There is no point replacing one of your melees with a different caster, depending on fight mechanics, it could cause things to not be predictable, among other things, however, in the same vein, you aren't going to replace the BLM with a different caster. DPS is king after all and i have already stated that a job utility can only do so much.

On the subject of utility, if all jobs in a given role do NOT have the utility (res being a small exception), then that utility is not something they balance fights around. They do not want to make a specific job mandatory after all. Res is a bit different as it is a loss in damage, not just from the job being inherently weaker, but because you have to take time to res, but healers should be the priority in ressing anyway. So really, unique utility for a specific job is meaningless. This is why jobs tend to get similar tools, which is where homogeneity comes from. Which, talking about this, homogeneity has NOTHING to do with a jobs DPS or making the job easier. It all comes down to how similar the jobs are to each other. Tanks having a similar defensive suite for example.

Which brings me onto the last point. You say I treat this as, in your words, a 'science experiment'. Call it what you want, the only thing that matters is the facts. If this means it is treated as a science experiment, then that is what it is. If you want to make a change to how the job balance is currently, you need to critically think about what the repercussions of that change are going to be, both positive and negative. Like it or not, at the core, the game is just a numbers game. Balancing these numbers is an incredibly difficult task and a change in the wrong direction can have drastic consequences.

So, my question is, why do you think BLM has to do more damage than SAM? What makes you say that? What are the pros and cons to this change? How is this going to affect balance? Are you going to need to change other jobs to balance this? How do you plan to change them if that is what you want to do? So many questions from just asking a simple question and whilst the initial question only seems to look at Casters and Melee, it will also impact P-Ranged. Feelings do no come into this, just the logical steps which leads to a specific conclusion.