It's close to apples and pears, dear Alberich, apples and oranges even, but I can see where you're coming from and it's a point worth standing still by a moment.
These works coincide in as far as each is seminal within their own cultural context: where one changed the face of opera for ever, the other changed the face of the (modern) novel likewise, to be changed subsequently beyond recognition by, in Joyce's case, 'Finnegan's Wake' to enter a literary terra incognita where it still roams, all alone, as one of a kind.
As regards its impact on posterity and its own art form within that 'posterity', I feel your choice of The Ring to be unfortunate: Tristan had a far, far greater 'life changing' impact on the profession than did the Ring, unwieldy behemoth that it is. There then too sits the less flattering analogy with Joyce in that poor old Finnegan, intended to be the crowning achievement of a life time's work, became so hermetic unless the reader is prepared to make his way into Joyce's labyrinthine imaginings and linguistic fireworks, that not too many ordinary readers on the famed Clapham Omnibus simply bothered. They sat on their readerly hands, and Finnegan simply languished, unloved and unregarded, and very much unread.
The musical Clapham Omnibus has many a passenger with similar sentiments about both the Ring and Parsifal -- 1939-45 didn't help in that regard either -- so perhaps Wagner and Joyce, bizarrely, share a certain communality of fate in being disregarded in the broadest sense with specific regard to the works they themselves would have thought and wished to be the most revealing and universal of all..?
Given the 'desert island' choice, my knapsack would be filled with both Ulysses and Finnegan -- having read both multiple times with relish (and hard work in Finnegan's case
) but you'd have to come up with Tristan before I started to squirm about sacrificing one Joyce to be able to take Tristan into my boat to the island instead... :-))
All the very best, as always,