What strategy are you talking about? All that does is create a situation where you use a skill when it's best suited, if using it at all? Heck, even in the example you gave for DRG, what's stopping them from using the fire WS even when it's going to do less damage? Are we going to be using it in a combo whenever it's up, or only when it's the most ideal?
If that DRG is going to do 20% less damage on an ability, why are we even going to take the DRG over another job that won't have this problem, or even have an ability that would be better suited? There's better way to introduce mechanics to the job that isn't tying to the elemental wheel, which is either going to cause imbalance to take away from development time, or be so homogenized that it might as well not be there at all.
Take this ouit of a vaccum, how is that damage loss compared to other jobs? Compared to other DRG abilties? As I mentioned, you've either introduced a skill that have as ituational use and has no other use other than to sit there if it's not feasible, or if it's going to be used anyway, it's doing less damage than other jobs who do not have this issue, or moreso if they have a stronger element.
By design, fights can be more variable and less monotonous than a job's gameplay, which is persistent throughout the entire life. Not just in MMOs but games in general. You approach boss fights differently because they have different patterns, but you have relatively the same tool kits. The longevity of class design is just as limited as any other content, but at least with fight scenarios they can be different from each other (much like how jobs are different from each other, for the most part anyway). When you start introducing gimmicks like elements, you need to design fights with that in mind, which limits what you can do. Back to the WoW example, it was less of an oversight bcause those sort of niches worked when you have 39 other players with you in the party. You were much more likely to have sources of frost damage. It definitely wouldn't work as well in a 10 man raid when there's no guaranteed you could have a mage in the party for that frost mechanic, and the oversight with fire immunity comes more into line when they started adding elemental attacks to other jobs that wasn't a caster, such as survival hunters having explosive shots, but by that point the content has become so outdated (2 expansions) that it didn't matter much.
It's not the case for FFXIV when we don't have a talent system for the jobs (since armory fills that nicely), but in context of equipment and weapons, you still wouldn't want to hinder a job's performance for something as niche as elemental weaknesses/strengths