renting a domain is insane, basically they are inevitably going to have to pay more and more rent if their business succeeds
I've honestly never herd of anyone Renting a domain, other than paying their yearly fees to ICANN.
Sounds ridiculous to me. Domains are basically the name of your business. Like you said, if the business succeeds then basically the person you're renting from has you by the balls. Since you won't even be able to inform your patrons that you will be moving if they jack the price up too high, the renter will just change nameservers and you're pretty much SoL.