Almost nobody does that, or even knows about it. A user will have to acquire their sim pin, before they can change it to their own. Most users have done away with the card the sim was attached to that had the pin, or got their phone with sim already installed. And if a user does have the pin and changes it to their own, if they forget the pin, then they have to get a new sim to order to use it in another device. There's also x amount of pin attempts. We are talking about the general public here.
Yes e-sims will be exploited just like everything else. But it won't be so easy. Right now is too easy for even low IQ opportunists to commit fraud by stealing a sim.
Cloning sims is still a thing. Obviously not as widespread as before.