The pursuit is not exactly fruitless, is just that the results will be underwhelming.
As I said, men cannot be molded back once their identities have settled. When that happens—if ever—we cannot say, some retain the personality they developed when they were 13, some keep their minds unbound. The first group are slaves, they will forever be trapped in their mindset, no matter where they find themselves in. Leftism is based off this way of thinking from those who are marginalized (and ruthless capitalism is from the master's view), so, it is only natural that all leftists who are deep into the ideology are complete slaves.
Even if we eliminate all marxist ideas and rationalizations, we will still get a defective human, fill his head with right-wing ideas and you will get an angry failure who now blames jews and women for all his problems, instead of wasting his time in discussions about unions and intersectionality, he would be talking about circumcision or how a Hollywood movie is literally White genocide. Fascism is abstract, unbound, and esoteric; it will never be understood my these kinds of people, now should be insult its name my pretending it to be possible.
And while unrelated, I want to add; this does not only apply to leftists, manny so-called « third positionists » also suffer from this. When using a spectrum to gauge someone's ideology you may have imagined that not having strong opinions about anything will put you on the same category as having strong opinions which are all over the place. This will risks categorizing Fascist men alongside slaves of fringe ideologies, be aware of the angry sheep in wolf's clothing.
The only thing I can see some future with is to use these converts as golems, they are already empty shells, no matter what ideology they use to paint their excuses. But having those people stay on the left and let the degenerate them from within is also an option, we would have to consider whether their long-therm corrosion of the left would give us a bigger advantage than to use them as golems.
>tl;dr: Instead of trying to convert them, we should be taking advantage of them