My assumption was that the ship sped through the Supremacy before it entered hyperspace, so its the added force (and energy) of the impartial jump that cuts through the Supremacy like a lightsaber. This is Hollywood science, so that made sense to me.
Yeah, I dont get the argument of why wouldnt they always do this? Holdo was desperate, it wasnt her original plan, and she expected to die anyway when the ship ran out of fuel. And its doubtful the Resistance would have the resources to build giant space torpedoes to do the same thing. More importantly, not every culture likes suicide attacks (and some consider them to be wicked acts), and Id imagine the Resistance wouldnt necessarily think to do such a thing. In fact, its something Id actually expect from the more fanatical First Order (and it would have been nice if Hux knew what she was doing because it was a FO tactic).
The bigger issues are the fact that no one recognized that the ship was close to a planet (despite supposedly being in the middle of knowhere), and that Holdo's sacrifice is undercut by Rose knocking down Finns attempt to do the same thing.
It was an uncharted planet and very few people knew there had ever been a Rebel base there. More importantly, the Raddus was the only protection the Resistance. and even a hot head like Poe thought evacuating the base ship was insane so there is no reason why Hux would have considered it as an option for the rebels.
Holdo's sacrifice is not diminished by Rose's actions. Holdo gave up her life because she cared about Leia and her comrades - there was no other way to save them. Rose cared about Finn and didn't want him to die, let alone for nothing - was hat rickety little craft really going to have any impact against the cannon?