A cultural tradition is not a prescriptive body of rules, but a practice of dynamics and principles, in terms of intention, craft, aesthetics. Also, it has a character, in the way a person has a character, which may change and develop, but which will maintain its outlines and its nature. So, in a cultural tradition nothing is 'conserved' - a context, a spirit, a 'soul' of an art form are not 'things' that need 'conservation'. The aesthetic and artistic dynamics that inform cultural traditions are living processes, reinventing themselves with every work, exploring unexpected potential, and reflecting the infinite variety of human nature.
With this I ... basically agree ... but that's why, in practice, I think it's thoroughly feasible to compose sonata forms that use blues, jazz, country and ragtime materials. If sonata is not a prescriptive body of rules but "a practice of dynamics and principles" then it's easy to imagine that the spirit of sonata as a way of developing material can be applied to blues and jazz and other kinds of popular styles.
Unless, of course, what is really meant by the cultural tradition of classical music is a very specifically defined range of instrumental idioms that evolved from the 18th into the early 20th centuries.
Theodore Adorno, in Current of Music, wrote about his frustration with American attempts at music appreciation programming. He strenuously objected to the idea that Haydn "invented" or "standardized" what is still called "sonata form". Adorno objected that what Haydn did was crystallize a way of developing and building upon musical materials and idea. What Adorno insisted that Haydn did not do was standardize the forms of the symphony or the string quartet. Adorno being Adorno he tended to believe the evils of monopolistic capitalism were to blame for a tendency to think of music as having forms that can in some way be mass-produced or explained at a purely schematic or template level. As basically wrong as I think Adorno was about all kinds of things, on this particular point, I think he was completely right.
There's much to be said in favor of such a polemic, really. I happen to love the music of Haydn and I would say Adorno was right to say that American music appreciation programming was wrong to speak as though Haydn "standardized" forms. There are few composers who are more likely to cheerfully break the rules that people attempt to impose on what he's written in his scores than Haydn.
And ... as Charles Rosen put it in his by now fairly standard work on the evolution and history of the Classic style, Haydn crystallized a musical idiom in his late string quartets, masses, oratorios and symphonies that was a genuinely popular style that was also brimming with learned technique; a similar fusion of the values of academic art music traditions with what Rosen called the virtues of street song was also, Rosen insisted, present in Mozart's works, too.
Where Borstlap and I seem to disagree is on the question of whether such a successful fusion of "high"and "low" musical idioms is possible since the 19th century. He seems pretty certain the answer to that question is "no" while I say the answer is "yes". The answer is "yes" if we understand the practice of dynamics and principles properly. In other words, if we understand that in Haydn's time he was composing in a popular and accessible style in which certain syntactic scripts that were later identified (somewhat wrongly) as forms were not really standardized or fixed. The paradox here is that, on paper, Borstlap would seem open to the idea of such a dynamic interaction of traditions that, in practice, he pretty consistently rejects.
So ... it's possibly ironic that Borstlap and Adorno agreed on something ... perhaps it could be proposed that, as a colloquial saying has it, a broken clock is still right once or twice a day (depending on the kind of clock is is).