Vendor Lock-In
Also known as: platform lock-in
The point where leaving a platform costs more than staying on one you have stopped liking.
Lock-in is not a clause in a contract. It is arithmetic. If moving twelve client sites means twelve rebuilds, you are locked in whether or not anyone used the word.
It builds quietly. One site on a platform is a choice. Twelve sites on it is a position, and the cost of changing your mind grew while you were busy being successful.
Some lock-in is a fair trade. A hosted platform doing your security updates has earned something in return, and for plenty of shops that deal is correct.
What is not fair is being surprised by it. Work out the leaving cost before the fifth client lands, because that is when the number is still small.