The idea is that if you choose a good enough icon, you
won't confuse people who know the old one. Users are at this point used to encountering unfamiliar icons and trying to quickly guess what they mean, so if a sufficiently communicative icon is chosen there should be no problem.
Importantly, the user has no reason to directly contrast the new icon to the old one. The user doesn't answer the question "Is this icon as effective as that old one?", they simply have to answer the question "Do I know how to perform the action I want to perform?", and as long as your save icon clearly communicates its meaning, there shouldn't be much/any confusion when the user tries to save. They'll look for something that seems to say "Click me to save", they'll see your icon, they'll say "Hey, that looks like it means 'Save'!", and they'll try it. (Aside: I don't say this randomly, I'm speaking from experience here; there have been plenty of applications in recent years that have tried out new "Save" icons, and I can't say I've ever had a problem figuring out how to save with any of them.)
As far as what "the point" is in changing out the icon, the point is that the entire reason for using action icons on buttons, etc. is to give the user an intuitive sense of what action will be performed when they click it, and as time goes on, the link between the floppy disk and digital storage will become weaker and weaker. And while it may be true that we could drag that symbol with us by convention, my question would be, why bother? If we can come up with something better, especially if we can find something that isn't tied to any specific technology (and I'd argue that we have), isn't this an improvement? I can't think of any advantage the old icon has over new ones other than the small advantage that it's familiar, but, as I said above, I don't think that's enough.
In other words, instead of asking "Why should we get rid of the floppy disk icon?", I honestly think the better question is, "Why not get rid of the floppy disc icon?"