It's not that there's 'nothing stopping' the airline, it's that under the normal operation of an airline, planes are moved around and rescheduled all the time,

It's not that there's 'nothing stopping' the airline, it's that under the normal operation of an airline, planes are moved around and rescheduled all the time, up just hours (or sooner!) before the flight. I would guess something like 0% of assigned tail numbers (not plane model) remain the same a week before booking. Airlines that fly multiple models on the same route would be harder to reassign, but days before the flight if it hasn't sold out, it's still very possible they'll get moved around. The safest way to not fly a particular model of plane is to not fly with airlines that open them.