I think the biggest counter-point for the opening up more group options is a few years back they had a data point saying adding more tanks doesn’t actually increase tank players; it’s almost entirely existing tanks switching over to the new tanks (And I think this phenomenon is replicated in other games like OW and League). As for healers equivalent to tanks, I think the hardest part is raids require from 3 up to 5 or even 7 healers but always take two tanks.
Because of the different ways to balance DPS (which, granted, they do not always do very well; just looking at the kit gap between rogues and rets) they can always add more DPS whether melee or ranged.
I do think in a vacuum a four spec class is the best option. I actually think your point on there’s always going to be a best and it’s not worthwhile designing, let alone balancing, around the highest of high end is extremely valid. I think they do too much of that as it is. The boring point is subjective for sure and I should’ve done without that. I think, if one had to be picked, adding annihilation over fortification is healthier; especially if the DF leaker was right and next expac features a titan four spec class. Supposedly reworking tanking is on the docket and I wouldn’t be surprised to see that as a feature for 11.0 as it is the part of the trinity which has evolved the least over time. This tracks with not adding a new tank until that rework happens.
However I would say if the reports are false, and next expac or two does not add a new class with a tank, I’d be more in favor of Evoker getting one even pre-tank rework.