Are you talking about the lolDRG/floor tank DRG memes that started in ARR and HW? Trust me, they had nothing to do with this supposed lack of utility you seem to be under the impression the job had, and everything to do with bad players (and with DRG’s unfortunate lack of magical defense in ARR). Quite the contrary, it has offered party buffs since HW when they introduced Battle Litany. And it was a staple job in both HW and SB. Any raider worth their salt knew this. Even the casual raiders who didn’t subscribe to the raid meta were well aware the holy trinity for DPS were DRG, NIN, and BRD in SB. In HW, it was DRG, NIN, BRD, MCH.
I think you have your expansions confused. No decent PF was locking out DRG in SB. It was part of the meta comp. SAM was far more common to be locked or passed up when it came to groups that were meta- or parse-centric because they offered nothing in an environment where buffs and buffing personal damage were king. Aside from the fact that DRG offered incredible synergy with perhaps the most broken DPS job in SB after SMN: BRD.
Are you perhaps confusing SB (Stormblood) with ShB (Shadowbringers)? Because those were two very different expansions with two different metas. Again: please do some fact checking yourself.
Re: BLM switching to RDM—this has always been primarily a prog strat simply because RDM offers the recovery option with Verraise, and BLM requires extensive fight knowledge so that they can perform their rotation with as little forced movement as possible. Prog strats will always take an option for a combat raise over not having it at all; as well as just simplicity in general. You see this with WHM and AST: WHM is taken for prog due to ease of play compared to the latter (no card shenanigans) and (in SB and ShB, at least) its bottomless MP and penchant for recovery. AST is used after prog has finished because of the buffs it brings the raid.