Not all of the left hated Obama. But much of the far left certainly did, because he was young, non-white and not from their handpicked list of saviors from the Bush years, many of whom were either has-beens or journalists. The attitude was and remained 'who does he think he is?' - to the point that a lot of those 2000s-era superstars have now drifted to the right or so far left that they might as well be right.
The reason they attack O'Rourke - or Harris, or Booker - are for much the same reasons: Those candidates are not their chosen saviors or superstars, i.e. Bernie. Anyone who challenges Bernie's shine is to be crushed. I don't think it'll work as well as they hope, not after everything the public has seen about Bernie, Jill Stein, etc.'s roles in 2016 in the last two years. They're also extremely alienating in their methods. But I could be wrong. (There's also an obvious racial/gender animus re: Harris, Booker and other female candidates or candidates of color, just as there was with Obama and Clinton.)
I don't think any next Democratic president is a guaranteed one-termer should we win in 2020. I think people could've easily claimed that about Obama going in, but in the crunch year it was never close IMO. It depends on a lot of things we can't know yet. And 'bothsides-ism' is under more public assault than ever before thanks to the rise of social media, which is good for the future, if anything right now is.