There are a huge number of peoples and places that are unengaged by the Gospel.
The answer is often cited as "Pray for workers for the harvest." And this is right - we should pray this prayer. We are commanded to (Luke 9).
However, the idea that sending "enough workers" to take in the harvest can be a mistake. "Enough workers" is often measured as, for example, "1 worker for every church" or "1 team for every people" or "1 team for every place" or some such.
The reality is, the number of workers we can send is limited by (1) the pool of workers and the available money but also (2) by the fact that some places we can't send workers to.
There are some places within the unreached world that we simply can't get workers to.
We've heard Brother Andrew's famous quote: "There's no country you can get into if you don't care about getting out." (Or maybe this was Greg Livingstone? It's passed into missions lore.)
In a sense, it's true. But methodologically it's flawed. Some places you, as an expat, cannot physically get to - you will be stopped on your way there. Some places are so dangerous that you will likely be killed. The point isn't getting the worker in, like a game of "capture the flag." The point is getting the Gospel there on a sustainable basis. If the worker cannot stay (or go in multiple times, perhaps), it's pointless. Any ant can get into a house. But as soon as he's seen, he may very well be squished.
Further, if we were to flood a lot of expatriate workers into the 10/40 Window, what would be the result? People would notice. It's not just white Westerners, either; Koreans got in trouble with mission efforts in Afghanistan, such that governments got involved.
The answer isn't the number of workers we send - that's a "vanity metric," something we measure for pride.
The answer is to send "enough workers" with a workable strategy that starts a Gospel movement in one place, and from that place seeps (via local workers) into the places the outsiders can't get to.