You can have every dps spec output the same total damage and still maintain the uniqueness of each spec. Everything you are talking about are individual abilities. There are not rules dictating those abilities must rank in any order or even output any given amount of damage. You can set a baseline of X amount of total damage for Y amount of time (since base-lining off of any given fight would include too many variables), which every dps will do at max level, unbuffed.
From there, buffs and gear affect that base line output. The baseline itself is like a pie with each ability for a given spec being allowed a percentage (or slice). The devs can decide how much each ability will output of the total, but every spec will output the same total, until buffs and gear come into play.
All of those unique class abilities that mitigate incoming damage, buff outgoing, etc, still matter a lot and keep each spec relevant, but the total damage is consistent across the board for all specs, before gear and buffs. So, taking two players playing a frost DK, you could very likely still get different total “buffed” output because each player is likely to gear differently and might even focus on different spell buffs (it happens). There would still be an ideal for any given spec, but everyone would have the same potential for damage output, instead of this farce of “class balance” that Blizzard has been running for two decades.
Now you’re shifting your argument to a different statement. How do you not understand what “all things being equal” means? Not to mention, you quoted the above statement out of context to try to make yourself look correct. That’s fine, since my words are there for everyone to read, but it makes you look foolish.
No, it wouldn’t. Setting a baseline just means every dps spec has the same ouput if “all things are equal”. Once you begin stat buffing, whether gear or otherwise, things change and individual play style and priorities take over.
Additionally, it’s already boring beyond belief. There are specific talent builds, buffs, group makeups, that every group (especially in high-end content) wants and some refuse to run without those specifics. So, all that blabbering about variety and uniqueness is bull because the current system is bland and lacks any sort of option for variety due to how people see the importance of “the meta”.