Given the way transfers work, that would also mean you couldn't move a domain from one registrar to another. So if you had a domain that anyone else would want, and your registrar decided to raise their prices or change their policies, you'd be unable to leave without losing your domain.
Domain registrars that engaged in holding domains hostage and increasing prices would probably eventually go out of business.
Even if that ended up not being the case, we'd have to fix how domains are transferred between registrars - same as how telephone numbers can be transferred between telcos but you can't sell your telephone number to someone.
