Just because a country is in the 10/40 Window doesn’t mean it’s uniformly hard to reach. There are (at least) three categories of countries (and these likely apply to peoples and cities as well):
Small populations, difficult to enter: places like Afghanistan, Libya, Maldives, Saudi Arabia, Somalia, the -Stans, and Yemen. These are relatively small population sizes, but their borders are tightly defended and the smallness of the population makes it easier to monitor them. These can be considered among the “last lines” for the Gospel to penetrate–it is difficult and dangerous to do so. “Reaching closure” in these places will require perseverance, prayer, and creativity–and if we’re honest, most of the people in these places may be “unreachable” at the moment insofar as human eyes can see.
Moderate populations, easier to enter: places like Chad, Turkey, Egypt. It is not easy to get into these countries, and some are harder than others (some even bordering on category 1). Still, their population tends to make them “larger markets” which makes them at least slightly more open to the world. Better still, many of these have fairly direct ties with the Category 1 countries listed above.
Very large populations, ease-of-entrance varies: places like Indonesia, Bangladesh, India, China. The bulk of the unevangelized individuals are in these countries (simply by virtue of their large populations). It’s fairly easy to enter as a tourist or on business, but it’s far harder to remain in place. Strategies to reach these places will need to scale, and hundreds of movements to Christ will be required. These places are home to many internal sociopolitical and ethnolinguistic barriers the Gospel will have to jump across.