There are probably a hundred other similar platitudes you could reel off in place of what's the worst that could happen. Springing to mind is nothing ventured, nothing gained which is at least a little more positive. However, most intelligent people recognise platitudes for what they are, and they are all as meaningless as each other.
Fears are often of the unknown. Your specific example, public speaking, is a good one because this does get easier with practice. There are practical steps you can take to be more prepared, and therefore more confident. Rather than use generalisations and platitudes, tackle specific fears directly by researching them and learning how to overcome them.