They haven't made some of them "quite useless" on some bosses. Every DPS is viable, some are a lot better than others. Your constant hyperbole is unwarranted as it only takes a brief look at warcraftlogs to see that every DPS is viable.
This is 75th Percentile for Krosus:
https://www.warcraftlogs.com/statistics/11/#boss=1842
This is 50th Percentile:
https://www.warcraftlogs.com/statist...842&dataset=50
Finally this is the Max:
https://www.warcraftlogs.com/statist...42&dataset=100
Note the lowest dps there is 726k logged by a Shadow Priest, while the highest is 924k logged by an Assi Rogue.
Notice that, Survival Hunter has the lowest representation but has logged 793k dps.
Notice that, all of the dps. Every single one of them. Is capable of far and above the requirements for the raid.
I would stop guessing if I were you, because it isn't me that lacks understanding.
So you're saying that every spec and class deals a different amount of damage? This changes everything. This must be new to t19 right? We've never experienced this before?
Not just viable, definitely realistic and obviously not suggested. Why would you suggest anything other than the best - that would be like suggesting take out from McDonalds over a Michelin Star dining experience. You're getting fed - you'll save money, possibly even enjoy the food but if someone is asking for suggestions they probably aren't asking for you to suggest from the bottom of the list.
It sounds like people are encountering a lot of frustration over not having the ideal composition for the raid. They're then blaming their failures on discrepancies between their own successes with other classes, specs or players rather than what they are - failures of execution or lack of gear/traits. The lowest logged (highest) DPS, 726k by the Shadow Priest in the data I linked to above, is more DPS than what is required. If you're bringing a raid full of shadow priests you can still succeed. Chances are though, that you have a good mix of DPS including some at the top.
Clearly, Nighthold is more difficult than EN. Hopefully Tomb follows suit.