Some very good friends of mine just released an email that said their time on their current field was done, and they were returning to the United States. Interestingly, however, the email was very particular that their engagement with their field was not finished - only their location was changing, as a result of changing tactics.
In my personal experience, and from the reports of others, it is often the case that when missionaries "return" from the field to the United States, people assume their missionary involvement is finished. Giving drops off precipitously.
The reason is clear: we in the West often have in our minds the idea that "mission" equates to "presence"; short-term mission means a trip somewhere, and long-term mission means we reside "there."
When we get into the messy details of missionary life, however, this is often the case but not necessarily always the case.
Long-term presence is needed for language and culture acquisition, it's true. "Newbie" missionaries shouldn't think they can do what experienced missionaries (who have learned the language, the culture, built up relationships, spent time, etc) can.
We should not equate "presence" with "engagement," however. We can see this in our own local spheres of influence - just because we are living in a neighborhood does not mean we know the people in the neighborhood, are engaged with it, etc. And just because we are not "present" in the neighborhood 24/7/365 doesn't mean we can't be "engaged."
Strategically, being present is not a goal but a method - a very important method, yes, but not the "success line." And sometimes, to reach the real goal (e.g. a planted church that can be sustained over generations and is scalably multiplying itself until everyone in the area/people/segment/whatever has the chance to hear the Good News and choose to become a disciple of Christ) may be reached by not being present. Our presence can be tactically dangerous, disruptive or delaying.
Remaining engaged with a segment is difficult from afar, yes. But what we really need are people who are full-time engaged with seeing the Gospel planted in a place ("Give me Scotland, or I die") whether or not they are present. (And, for that matter, supporters and investors and helpers who understand that vision and goal, and remain supportive whether or not the missionary is on the field or not.)