My present motivation would be Role Playing -- which sounds like a universal get out clause but I don't believe it is. I mostly agree with your first paragraph. I do believe that Skyrim offers great diversity in builds. I also mentioned that player styles and tactics are important factors; however, I don't agree that having equal offensive and defensive capabilities is not an illusion of choice.
Mages and warriors having
potentially equal offensive and defensive capabilties does not mean that the difference between them is illusory, that the choice between them is illusory. A choice between options which are merely potentially equal is not an illusory choice. It is at worst potentially illusory. In the instant case, it is not even potentially illusory, because the chooser has complete discretion as to whether the potential equalities become actual equalities.
The perilousness of an argument that potential equality necessarily vitiates real choice is revealed by consideration of the fact that every character is, at character creation, potentially equal to every other character in every single way, from name, to race, to perks, to equipment. Are the choices you make there illusory?
On top of all of that, there're very real and obvious differences between inflicting X damage with Flames and inflicting X damage with a battleaxe. So even if there was potential equality in terms of damage capabilities between mages and warriors, there's no qualitative identity between the categories.
A Destruction only Mage (assuming here that you mean that you only spec that perk tree) may not be a viable build and would require supplementary skills and perks. But then again the same thing can be said of a Warrior that only perks the one handed tree. Most builds need supplementary skills to be viable so I see no issue here.
If players found that for warriors to remain viable required a playstyle they didn't enjoy, I'd say that they had a completely valid grievance.
But to steer myself back on topic: the viability of high level Destruction spells (ignoring strict equations) comes down to the subjectivity of an individual player. That subjectivity is influenced by many things; prejudice, prior expectations, confirmation bias and play style to name but a few. One player states that they are pointless while another explains a tactic that renders them all powerful. All players are not created equally -- if you'll excuse the perversion of that quote.
I totally agree that it is subjective, I'm not trying to say that destruction mages at high levels are objectively not fun, just that I don't find them fun (and that I want to). Obviously, game designers can't please everyone, since we'll have conflicting interests, but I think that increasing player choice is the best way to maximise the satisfaction of our preferences. Let the players choose. At the moment, there simply is no choice that satisfies those of us who want to play destruction mages but find stunlocking boring. Well, there is for those lucky few of us on PC, but plenty of people are playing on consoles and quite a few seem to have the same view of vanilla destruction mages that I do.