It's good to see DRK being utilised in WF clears. It's been fairly common to see people lock in PLD/WAR slots in PF groups this expansion, even after the previous Ultimate clear. Anything that promotes comp diversity is a good thing.
I think that 'viable' is a misleading term. FFXIV is a team game, such that anything can be viable so long as the rest of your team covers for your weaknesses. A tank without raid-wide mitigation or self-healing is still viable. You just shift the burden to your healers. A tank with a lower damage output is still viable. You just shift the burden to your dps. A tank with weaker enmity generation or mitigation is still viable. You just shift the burden to your co-tank.
Every time that you shift the burden to someone else, you place an additional constraint on your team (in some cases, it even places limits on what other jobs you can bring). This becomes especially important as a tank, because how you play directly impacts everyone else's performance. A self-sufficient tank can pick up a lot of slack for the rest of the team. A tank that is overly dependent on their teammates to function becomes a burden.
Some people try to divert the discussion away from balance in favour of viability with the claim that balance only matters at high skill levels. This is false. Because casual groups have a bigger variation in player skill across the group, cross-compensation becomes even more important. Your 70th percentile tank's dps is crucial to offset his friends 20th percentile dps. If your healer is a bit uncomfortable with the timings, savvy use of tank raid mitigation can prevent a wipe.
I've tabulated dps differences between the tanks several times before. Because DRK has relatively weaker burst, the gap between the tanks narrows slightly with higher percentiles and better uptime. But the average player isn't three standard deviations from the mean.
Gameplay is just one of the issues facing DRK. There's a fairly sizeable list of them on the front page of these forums. There are issues with burst, overly situational abilities and stance locks on a job without freedom to stance dance. There are questions about job identity. Individually, you could probably pick any one complaint and provide a work around. Have your healers saving their cooldowns for Living Dead. Have your dps push out more damage. But they all add up, and it becomes especially difficult in groups with a lesser degree of coordination.
Most opinions on balance and job adjustments are subjective. There's been a trend of late for people to try to "theorycraft" their opinions into something that looks objective. But the collective opinion is what matters. That in turn influences the jobs that people pick, and also the jobs that they want to bring in their groups.
The real goal should be variety. Bring a NIN for trick and slashing. Bring a DRG for piercing. Bring a WAR for slashing, high dps, and utility. The reason why some jobs end up being a consistent staple in so many comps is because their job identities have been grandfathered into some sort of powerful raid benefit.
What would I like to see? I'd like to see players running radically different comps in 2019 compared to what they were in 2014. This is the same discussion that we were having at the end of Heavensward going into Stormblood. You need to identify why certain "niches" have consistently higher pickrates and address those points. That's the real goal of balance. To encourage diversity.