I can see what you mean but I also disagree with you The Progressive Movement was based on making music that was "progressive" in the true meaning of the word. Musicians tried to expand the limits of popular and rock music with influences from other musical genres. After the first wave of the Prog Movement, the term "progressive" of course became a definition for music that sounded like that by the bands from said first wave. Musical subgenres with strange terms like "Retro-Progressive Rock", which in itself is already a contradiction, evolved and bands wanted to revive the kind of music that Genesis, Yes, Crimso, VdGG, Gentle Giant and all those bands made from 1969-1976. That is not what "progressive" stands for. Therefore I think one should make a difference between the term "Progressive Rock" and making music that is "progressive" in the true sense of the word.
And yes, it is possible to be progressive. Take Robert Fripp for instance. He always disbanded King Crimson in the exact moment when he felt that their new way of making music became a concept or subgenre in some way.
Others are Frank Zappa or David Bowie. They always did something new, for their entire career.
There is no such thing as the "Progressive Movement". Musicians didn't just get together and decided to make progressive music. It simply happened as a natural evolution of music, the same with any other genre. Like you said, they started mixing rock with jazz, blues, classical music, etc. So the term "Progressive Rock" was invented to describe the bands who played such kind of music. And like I said, this idea that you have to reinvent yourself constantly to be progressive didn't exist. It was simply used to describe that kind of music. Nothing else. But this idea keeps spreading. I think somehow people get confused about the genre and too attached to the word "progressive".
I don't think Fripp disbanded KC just because he didn't want to fit into a subgenre or to repeat himself.
As I said in my previous post, it is impossible to make "progressive" music in the true sense of the word. You can progress only to a certain extent. Yes, Zappa, Bowie and many others reivented their sound constantly, but you can't escape who you are. Your signature and identity is still in the music, it doesn't matter what you play.
If you have to reinvent yourself constantly, then there is no Progressive Rock as a genre. DT and all other bands aren't progressive. And the ones who reivent themselves are only progressive to a certain extent. Musical genres, musical notes and melodies are limited. If you could live 5 thousand years and you released one album per year, someway down the line you would repeat yourself. Then you wouldn't be progressive anymore...
It really doesn't matter how much you try to be authentic and create something new. You have influences, inspirations, you are born and raised in a certain environment and that shapes who you are and what you will play. Whether you listen to In the Court of the Crimson King or The Power to Believe or Red, it doesn't matter. Fripp is Fripp, he has his style and you can clearly recognize his playing. If you could change who you are and transform into a new person constantly, losing all your memories and you experiences, then, and then only you could be progressive, but, again, only to the extent that music allows you.