Where are people saying that Lenin/Marx were infallible? The outlined the existing structures of power incredibly well. It’s not like they’ll always be relevant, just in our current time, with the current power structures, their model is still incredibly applicable to revolutionary movements.
Of course they won’t say it, but there’s a persistent undercurrent of stuff like “you’re not a True Leftist unless you’ve force-read Das Kapital” that accomplishes the same thing. No science in the world places such emphasis on a 150-year-old text, and rightly so.