It seems to me that we need to decide what we mean by "the Mecca". My own sense (not that that means it's right, and it will no doubt mean different things to different people) is that it's a place you have to visit at least once in your life to satisfy an imperative in your belief system.
So, for me a "Mecca" would be a place I've never yet visited, which has some sort of mystique. My current answer would be South Asia, which was the plan for this year - Bangladesh, Bhutan, India, Nepal, Sri Lanka - I've never been to any of these (other than airports) and they offer a fascinating opportunity to learn about their history, culture, music, food, architecture, and art. However the pandemic has put this plan on hold : (
To seek to answer the implicit question of where would be the Mecca for most people - that's a different question, and difficult to answer, especially given the range of opinions on what "the overland" means. In terms of my appreciation of the term, I guess I'd be hard put to choose one region. South America is a fascinating region in which to explore with a vehicle, and it's fairly easy to do so. Africa (really at least half a dozen distinct regions) offers an image of remote and exotic travel, and certainly satisfies the "mystique' qualification. Europe offers huge range of countries and ecosystems, however there are fewer opportunities for the "remote travel" which seems to be a requirement to qualify as an "overlanding" destination for many. Australia is a country in which "overlanding" is a way of life for many, and there are many areas to explore, with low population density, satisfying the remoteness aspect. Asia is immense and varied so impossible to consider as a single destination - lots of "mystique" and in my case the current "Mecca" (as mentioned). North America also offers a huge range of ecosystems (from Chiapas to Newfoundland to Deadhorse) and could legitimately be viewed as a common "Mecca" for many.