It depends on the school. For one that has rolling admissions and is releasing decisions in the fall, I would guess that they compare responses to what they expected for that time of year and that they may adjust if there is a significant variance. It is new information, but maybe not information that is vastly different from expected. If it is, you are probably correct that they will adjust. But to be fair, we see schools every year that over enroll and we see others with lots of WL activity, so the dynamic modeling you credit admissions with doing still seems to have a pretty significant unknown component.
For a LAC that has only ED and RD and fixed decision dates for releasing those and a final date for students to reply with their decisions, no, I don’t think it’ll change anything. Those schools will go to the WL on the same date in May. The number of offers they extend at that point and what they want those students to bring to the campus may differ from year to year, but the process will not.
And as I said above, we have seen schools here on CC that have gone to their WL after all RD decisions have been released but before May.
But remember that WL offers in May, vs more RD acceptances, have a very short decision (24 hour verbal) turnaround so they are ideal for a school that is putting the finishing touches on its class. Many of the LACs have WL that are bigger than the whole freshman class (shame on them!) so have little difficulty in identifying the last 30 students who will round out the class as needed and they don’t have to worry about yield because the timeframe provides definitive information for them to work with.
So my point remains the same. The schools are prepared to handle waiting for your decision. When you are ready to give it, do so. But it doesn’t mean that some other deserving soul will necessaily benefit. They may have fully anticipated that for every three people like you who was offered admission, only one would say yes.