At the very least, they want to completely remake it from the ground up in their own image (which just so happens lean heavily on Marxist theory...another thing they're not shy about on their official website)... while they may not think it'll be destruction, history suggests otherwise.
One area where I'll give them kudos, as I noted earlier, they don't hide the ball.
They have a pretty polished flashy website where they list out their entire agenda and what they want to do.
It's the people who want to rush to defend them against conservative criticisms that are either trying to hide the ball on their behalf, or are simply ignorant of what their platform actually is.
"They just want to tax the rich more and foster more equity and have universal healthcare and a few extra free goodies"...uhhhh no, they want to do a lot more than that.
The very fact that they openly admit in bold print that their vision is to get a majority in congress via seat expansion in strategic areas, and then:
"We want to abolish the Senate and replace the President and Supreme Court with an executive and judiciary chosen by and subordinate to Congress...oh, and then we want to write a new constitution"
Shows that they're pushing for some sort of single-party controlled sort of deal. That goes quite a bit beyond "they just want to be like Denmark"-style defenses of them that people want to prop up.