I share your opinion, which is eloquently expressed in this post. I would even add that I believe her vocal difficulties were hastened by her crooning in the studio. Cornelius Reid mentioned in his book The Free Voice that constriction, which inevitably happens when you lighten the voice, can "de-coordinate" the registers. And in the end, I believe this is what brought Callas's downfall: the registers became separated from one another, hence why people sometimes talk about her "three voices". Nonetheless, she maintained her great artistry, which makes her vocal decline even more saddening and frustrating. Such feelings also inhabit me when I think of the evolution of Tebaldi's voice...Also, I don't think it's just later and earlier Callas, I think it's studio and live Callas too, just that she was more prone to "special" singing in the studio later on as opposed to her earlier studio recordings. To try and give an example of the kind of thing I'm talking about, I think it helps to compare her later studio and live recordings. Here's her 1956 (I consider all post 1953 Callas "later") Si mi chiamano Mimi:
It's musical I guess, but it's boring and kind of limp. I get no sense of the depth of Mimi's character and feeling. Instead I get what I hear as Callas undersinging in order to sound sensitive. There's "lightness" and "shading", but it sounds fake to me.
Here's a live recording made a few years later:
Suddenly you can hear a real voice start intone the music -- she has to use her real voice to be heard. The lower notes are stronger and there's what to me is real feeling instead of affectation. There are vocal issues (including some distorted vowels of her own, which, even when they are quite serious as in her late recording of Ma dall'arido, never receive the same criticism as Sutherland's), but the interpretation is very strong. The former has more shading and what not (and also seems like a lullaby to me), but the latter, to me anyway, has more voice and thus more possibility for art as well.
This analysis doesn't apply 100% to the Massenet of the competition (e.g., her low notes are very strong and dramatic in the studio recording, and her tone is less pulled back and pallid in part because of the context of the piece and in part because I think she identifies more strongly with this music), but I felt some similar frustration while listening to her rendition.