Well said, @CruCru.
As someone whose DD went through 11+ last year, just a couple of things to add.
Places relinquished don't always lead to offers from the waitlist. Last year, for instance, the uptake of places at WHS was so high that not only did they not go to WL but they ended up taking extra students above and beyond the bulge year (and extra class) already planned. All the schools we applied to last year barely went into their WL even though I know that we and many of DD's friends had given up places at those schools. The number of offers:places is far from an exact science so there is no guarantee that if someone gives up a place, another will become available. Every year there are posters on here asking if anyone is planning to give up a place at X school, assuming that a WL place will immediately become free, but sadly that isn't always the case.
I completely agree that it's right to give up places you know you don't want asap. I also remember that particular poster (it was their daughter, not son, if I'm thinking of the same one) and their self-justification and lack of thought for others was appalling.
However, I think there are some legitimate reasons why a family might want to hold on to a place until very soon after offer holders' days.
In our case, although were were 99.9% clear on our chosen school, DD felt strongly that she wanted to see most of the others again to silence that 0.1%. She didn't want any "what ifs". So we felt it was important for her to have a chance to see schools again - with offers in hand - and make that choice. It's quite different seeing a school when you have an offer in the bag from Open Days - you get to ask more probing questions, meet more teachers, and it's not horribly crowded like many Open Days are. Most offer holders' days are the week after half term, and we rejected the other schools as soon as we'd done the final visit, so with plenty of time for someone else to be offered the place before the deadline if WL places were going to be available.
So, yes, be mindful that others might be anxiously awaiting places but also do any due diligence you feel is necessary before making a final decision.
Also, just to say that every year for the past few years, I've known people without any offers at all on results day. But I don't know anyone without a school - for the large part, a school they were happy with - come September. Once the dust settles, and the offers deadline has passed, many schools do find themselves with spaces still available. Many prep heads will help in that situation, and many parents just hit the phones and do the work themselves. But it's not the case that come early March, every independent school in London will be full.
Good luck, all. I know this wait is awful, but you're nearly there...