What I don't understand is how Tony went many years picking really great keyboard sounds and then lost his touch in later years. For the first 15 years, he wasn't just sticking with old equipment. He would get new keyboards and then use some really nice new sounds. While the sounds he used on the IT album were very much of the moment, I think they were good choices and fit those songs (e.g., on The Brazilian).
On WCD I liked some sounds, e.g., the "elephant" and the backing chords in No Son of Mine, all of Fading Lights. And then he picked very underwhelming ones, like the whole instrumental for Living Forever.
His picks for sounds on the 2007 tour were also questionable.
RE: the first paragraph, I personally felt that his keyboard sounds in the 70's material was actually somewhat limited. Back then they were very defined: Varied mellotron sounds early on-mostly in the PG era but into W&W...the rock organs - those followed him from the beginning, straight thru Duke. Then there was the choir sounding chords for a few albums. As for the lead synthesizer sounds, those were also more or less the same, just programmed differently-with a flatter or brighter sound, different amounts of slide effect, etc. They were all used to great effect of course, but those were basically all of the sounds he used in the 70's for the most part.
Then in the 80's, the synth sounds became VERY much-as you mentioned-of the time, appropriate for the songs and again used to great effect, but I recognized many of those exact sounds on songs by other 80's artists. More to fill the space and accompany, than to create a separate part or solo.
Then we get to WCD. Different synth tech had been released, lots of different sounds and effects that hadn't been heard before. Tony utilized these new sounds very effectively, and IMO that's why WCD sounds like nothing they (or many others at the time) had done before. Some sound choices were hits, and some were misses, but IMO that is less about simply the choices Tony made with his synthesizers, and more on the choices of the band as a whole.
Your comment re: the Turn It On Again Tour synthesizer choices however, I do FULLY agree with! I always thought it was kind of lazy to use the EXACT same lead synthesizer sound for literally all of his songs (i.e. In the Cage, Firth of Fifth, Follow you Follow me, Ripples, Los Endos), and some of the background chords or organ sounds in the older material lacked a certain power that had been displayed with his choices on previous tours (i.e. In the Cage, Afterglow, Firth of Fifth).