Given the need to estimate yield accurately, a college that issues EA or early rolling admission offers presumably assigns a yield value to each admit. I.e. if a specific admit is estimated to have a 23% chance of matriculating, s/he counts as 0.23 student added to the class. If the student declines admission early, then s/he now counts as 0.00 student added to the class, so that the college now has 0.23 student spaces more left to fill than before the student declines admission. This may result in an applicant at the margin being moved from the waitlist to the admit group.
Of course, the reverse can happen if the student matriculates early, changing his/her yield value from 0.23 to 1.00, reducing the number of remaining spaces in the class by 0.77, so the college may admit fewer than it would have otherwise.
While colleges “overbook” based on assumed yield chance for each admit, this does not mean that they will ignore new information that affects yield (i.e. early admits matriculating or declining early).
However, the above is not really too much of your concern in your personal decision. Decline an admission offer only when you are certain that you will not attend.