TL;DR: no, the user ID is not changing anytime soon. Kontalk was designed this way and changing this would mean becoming another project.
There are a number of reasons why Kontalk was designed to be used with phone numbers.
It's an easy and common way of identifying a person. You usually have your friends/buddies phone numbers in your contacts. It's also easy to use the auto lookup feature and less heavy for the servers (remember Kontalk should be cheap also for server administrators, imagine if we would have to find matches for any user identifier in your contacts).
Less spam. Spamming on mobile numbers is much more expensive for a spammer than using a free system (free as in easily and anonymously accessible). Concerning privacy and anonimity: Kontalk is safe (as in privacy), but not anonymous. We knew from the start that using phone numbers would have meant sacrificing anonimity (see next point).
We aim to provide a free community-based communication system, easily accessible and usable by anyone. People feel natural to link their person to their phone number. Again, about anonimity: you can't have usability without sacrificing a little from other aspects of the design (in this case, anonimity).
SIM cards can be used with other devices different from mobile phones as well (think tablets, or 3G modems capable of receiving SMS). It doesn't matter: your number is your identity. Or maybe I'm not understanding your point in this question.
Another note on this: if the phone number is dismissed (e.g. expires) and then recycled to another person, the new Kontalk account will override the old one, throwing out the old account owner (since he/she is no longer the owner of the phone number).