^^ Colleges do not “offer a spot” to another student when one declines. They have yield projections and expect a certain number of students to decline. Iow, it’s not like a full parking lot where one car can enter only as one leaves.
It IS true that SOME colleges may go to their waitlists earlier than the final decision date (usually April 30) if the # of admitted students declining spots is significantly above their projections. But many of the schools will wait to see exactly what the class still needs after the final decision date before deciding who to accept from the WL. (That’s the luck part of this. If the dust settles and there’s a gender imbalance, the WL offers will reflect that.)
It is great to be considerate of others in the process, and by all means, let the school know when you are absolutely sure you will not attend. But if you are not ready to decline for any reason, there is no moral imperative to rush. The system will work fine as long as you stick to the deadlines.