So....this isn't entirely true, but granted 99.9% of the time, yes the ship will wait. However, there are could be other factors that could cause the ship to depart prior to a
DCL excursion returning: port scheduling, tides, etc. that MAY mean that the ship has to leave. However, if you're on a DCL sponsored excursion, DCL will pay to get you to the next port. I've only heard of it happening once. I think it was a British Isle and it was a tidal issue?
I think most of those people running for the docks aren't on a third party excursion. Every third party tour I've ever been on, I think the tour operator was more stressed than I was about getting back on time and they always had me back super early. I don't think I've ever even been remotely close on a third party tour. But I do a lot of research and I book reputable tour operators. I mean, it's their livelihood. Could you imagine if they got someone back late? It would literally be the end of their entire business. I think those running for the dock people lost track of time drinking over at Señor Frogs.