The composer has stated many times that he needed to move away from this style as he felt he had exhausted it
This is clear. But whether it was really necessary to go from there to a much more retrospective style is another question. I remember reading an interview with him from (I think) the mid-1970s in which he said he might go more deeply into electronic music, which would have been interesting, given the partly electronic soundtrack he had already composed for the film
The Saragossa Manuscript in 1965. Having said all this, much of his earlier work is quite derivative of the music of some of his contemporaries, Xenakis, Ligeti and Stockhausen in particular, sometimes almost to the point of plagiarism (Ligeti's
Continuum in his
Partita for harpsichord and orchestra, Stockhausen's
Momente in
Canticum Canticorum Salomonis etc.). This habit of his is something else I find puzzling about his work.
this belief that his earlier, more avant-garde works are somehow better just because they're wilder and more spontaneous is wrong-headed
It's a matter of taste, not of the rightness or wrongness of one's head, isn't it? Some of us might find wildness and spontaneity highly attractive as features of music. Anyway it's interesting that you find a wide range of style in the works you mention, and thanks for those recommendations, I do sometimes delve into Penderecki's later music to try to find something in it that I can get to grips with (rather than "writing it off"), but so far I haven't been successful. Going back then to the music from the 1960-75 period I feel much more at home.
This, 100%. He got 40+ years of works out of the tonal idiom compared to 20 years at most out of avant-garde.
And Paul McCartney got 8 years out of the Beatles followed by 50 years (and counting) from his rather less inspired later output. That's a non-argument really...