On foot, metro, bike, bus. Paris probably has one of the best metro networks in the world.
Its also one of the most sketchy and dangerous. If you hop on an escooter, you're not going to find a homeless dude there jerking off, there's no vomit anywhere, noone will assault your wife and you're not getting stabbed nor mugged. Can't say the same about the Paris metro.
I'm sure what you describe happens, but I've used the Paris metro everyday for years and never had any of those things happen to me, nor have I witnessed it happening to others.