RDM underperforming by the way of neglect

[RylaBee]

BLM has kinda transitioned from caster to “fake melee” that competes more with a melee slot allowing double caster to be a viable comp assuming one of those casters is a BLM

IMO, BLM did not "transitioned from casters to fake melee", it has always been there, exactly for the reason you mention.
If you look at top raiding dps for past last two expansion, BLM has always been among 1-4 top dps except from some short-lived period when adjustments screwed that. Admitedly, this was also because Ninja was regarded as ultimate support DPS, and was lingering at the bottom with Support jobs.

The issue indeed isn't BLM, but that gap between Melee(and BLM) and everyone else has become bigger.

DNC is handling this best due to symbiosis with the strongest dps in the party, which after all is their purpose.
This is true for BRD to lesser extent as well.

Meanwhile SMN, RDM and MCH have to mash buttons&pray to crit gods.

RDM specifically is affected additionaly by the fact, that its potencies has not been touched since start of expansion, while everyone else's were (often multiple times).

Oh hey, RDM HAS been touched ONCE since start of the expansion, in 6.1.:

"Displacement - Action animation has been shortened."

That is why I refer to RDM slide as: "by the way of neglect".