Is there a synonym of "transcend" that has negative connotations

The most appropriate word I can come up with is "devolve"

Devolve: to degenerate or deteriorate gradually: After several hours the discussion had devolved into a shouting match.

[American Heritage Dictionary]

Would “... in politics has descended from the once ...” suit your purpose?

One synonym of transcend that is almost always used in a negative sense is overshoot:

[Merriam-Webster]
1 : to pass swiftly beyond
2 : to shoot or pass over or beyond so as to miss

In other words, while transcend goes beyond something in a good way, overshoot goes too far beyond something.

Used in the example sentence:

The argumentative scene in politics has overshot the once epistemological purpose of rhetoric and debate, in favor of treating rhetoric as a tool, debate as a means to an end other than truth and with a focus on winning the discussion, instead of learning from it.

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Note that Merriam-Webster specifically lists overshoot as a synonym of transcend in the same sense as used in the question (I have emphasized the word):

2 to go beyond the limit of
   // a person who believes that any true understanding of God transcends human intelligence

Synonyms for transcend
break, exceed, outreach, outrun, overpass, overreach, overrun, overshoot, overstep, surpass

Of all those synonyms, overshoot is the only one that has both a negative connotation and sounds natural in the context of the example sentence.