There are some bourgeois sources that took the trials seriously, back when they happened. The first person that comes to mind is US ambassador Joseph E. Davies, whose book Mission to Moscow was turned into a Hollywood film that is fairly pro-Soviet. The labor politician Denis Pritt was also there, and took them seriously.

Very few bourgeois historians have actually tried to substantiate the claim that the trials were all completely phony. It really is an axiomatic thing, as poking around too much into the foundational assumptions of anti-communism only hurts them, so they simply dismiss the topic out of hand. The few that even address the topic at all simply point to Trotsky's Dewey Commission or the Khrushchev era rehabilitations, most just expect the reader to be familiar with the (forced) school reading of Orwell's fiction or Arthur Koestler's Darkness at Noon. What the anti-communists can't prove with facts, they simply fill in with fictional narratives. In fact, I don't think I've ever even read a bourgeois account of the trials that didn't explicitly reference Darkness at Noon, usually right at the beginning!

One of the things I've always found though, is that anti-communists always play on people's inability to understand that communists, and the Bolsheviks in particular, were violent revolutionaries, who essentially waged a decades long conspiratorial struggle against social-democrats, culminating in the overthrow of the social-democratic Kerensky regime itself. This aspect of Bolshevism most people simply do not understand, and hence it becomes confusing to people why communists would conspire against each other in the first place. This gap in understanding is then filled in with the cartoon caricature of Stalin as mad evil despot, and the rest writes itself. In reality, the knowledge that communists (at least the successful ones) are violent people that devote their lives to conspiring against governments, and that have developed an entire analysis on why their Left opponents are a bunch of agents of capitalism, opponents that most people can't easily distinguish between them and the communists, should give one a ready-made understanding of why communists would conspire to overthrow one of their own.

Instead, we're treated with the odd appearance of the bourgeois lionization of certain communists. Mad-devil paranoid Stalin killing innocent honest communist saint Buhkarin, says the bourgeoisie! Even their own literature on the topic shows they have never moved beyond "Left" wing resentment that their faction lost, which is the ultimate origin of all this nonsense.