It touches our primeval fear of what do we do when something screws up?
Yes you need to be prepared to spend the night at sea. Someone here ran onto the rocks trying to get into the anchorage at 10pm. Much safer to stay at sea.
The flexibility one needs is more than one night at sea. A couple of times we have had to sail on for a few DAYS because the place we had lobbed into was untenable for one reason for another. We did it on both occasions without a problem because of a mental attitude of flexibility.
Now onto the specific point of small anchorages... They are rarely fully full. You just get the bit at the mouth of the bay where its rolly. You get an uncomfortable night.
However, yes, you are right that theres some tiny anchorages - one classic Mediterranean photo shows some idyllic anchorage that when you look at it really only allows one boat to anchor but you know 1,000 boats must be trying to get in it each day. The rule for that is "you snooze you lose"! There's no way coming in at 9pm you are going to get the best spot, or in there at all!
in those great anchorages I get set anywhere I can and then each morning sit in the cockpit and wait till theres movement on deck of a boat riggggghhhhhttttt up the front in the prime position. As soon as the LOOK at their subber I up anchor for me and move in!
If an anchorage is so tight and you must anchor but know you are too close: drop the anchor in your best spot and dinghy over to the other boats around and say hi and explain the problem and that you will move in the morning.
And if someone else anchors near you DON'T stand on your deck arms folded gralring at them. You look like a ******** to everyone in the anchorage. Wave at the new people, Go below and wait for 30 minutes. Either they will work out they have parked too close or they will be asking /explaining. They might be really nice people worthy to meet.... just like you